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Adaptations of birds for life in deserts with particular reference to Larks (ALAUDIDAE)

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Birds inhabit all deserts, an unusual observation because they are mainly diurnal, non-fossorial, and have the highest mass-specific metabolism of all vertebrates. Many of the “adaptations” of birds for life in deserts are thought by some to be intrinsic in all birds, and thus have been considered to be pre-adaptations. Nevertheless, recent evidence suggests that desert birds do differ from birds of more mesic habitats, and physiological adaptations include reductions in basal metabolism and cutaneous water loss. Behavioural mechanisms that permit a desert existence include nomadism, enabling them to move away from low resource patches to more productive patches, and the ability to use features of their environment to escape from high temperatures. Desert birds can prevent overheating by using rodent burrows and other shelters during the heat of the day. Birds in deserts include a high proportion of ants in their diet, a resource not often exploited by birds of more temperate environments, and they also show seasonal or opportunistic shifts in their diet, feeding on seeds when available, and on more succulent green plant matter, such as leaf bases of grasses, after rain. Certain taxa, for example sandgrouse (Pteroclidae) and larks (Alaudidae), are mainly found in deserts. The adaptations in some species of larks are discussed in detail.
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Birds inhabit all deserts, an unusual observation because they are mainly diurnal, non-fossorial, and have the high-
est mass-specific metabolism of all vertebrates. Many of the “adaptations” of birds for life in deserts are thought by
some to be intrinsic in all birds, and thus have been considered to be pre-adaptations. Nevertheless, recent evidence
suggests that desert birds do differ from birds of more mesic habitats, and physiological adaptations include reduc-
tions in basal metabolism and cutaneous water loss.Behavioural mechanisms that permit a desert existence include
nomadism, enabling them to move away from low resource patches to more productive patches, and the ability to
use features of their environment to escape from high temperatures. Desert birds can prevent overheating by using
rodent burrows and other shelters during the heat of the day. Birds in deserts include a high proportion of ants in
their diet, a resource not often exploited by birds of more temperate environments, and they also show seasonal or
opportunistic shifts in their diet, feeding on seeds when available, and on more succulent green plant matter, such as
leaf bases of grasses, after rain. Certain taxa, for example sandgrouse (Pteroclidae) and larks (Alaudidae), are main-
ly found in deserts. The adaptations in some species of larks are discussed in detail.
79
Adaptations of birds for life in deserts with particular reference to Larks
(ALAUDIDAE)
BY W. R. J. DEAN1AND J. B. WILLIAMS2
1DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, University of Cape Town, Rondebosch, 7701,
South Africa. e-mail:lycium@mweb.co.za
2Department of Evolution, Ecology, and Organismal Biology, Ohio State University, 318 W 12th Avenue, 300
Aronoff Laboratory, Columbus, OH 43210, U.S.A
INTRODUCTION
Birds occupy the most inhospitable deserts on earth. In these
extreme environments they face two selective pressures,
extreme heat and aridity, but many of the adaptations of birds
for life in deserts have been thought to be “pre-adaptations”,
intrinsic to all birds (Maclean,1996). Evidence has accumulat-
ed that birds have evolved both behavioural and physiological
adjustments to the desert environment, and that previous
notions of “pre-adaptation” were based on insufficient data.
Many desert birds differ from birds of more mesic habitats,
particularly in coloration and behaviour, and less in diet, repro-
duction and moult. Behavioural adaptations include nomadism
(Dean,2004), the use of shelters to avoid the heat of the day
(Maclean,1996) and nocturnal courtship displays (Jensen,
1972). Physiological adaptations include low basal metabolism
and economy of water use (Anava et al., 2000; Williams, 2001;
Tieleman et al., 2002). Birds in deserts include a high propor-
tion of ants in their diet (Keith et al., 1992), a resource not well
exploited by birds of more temperate environments, and they
also show greater seasonal or opportunistic shifts in their diet,
feeding on seeds when available, and on more succulent green
plant matter, such as leaf bases of grasses, after rain
(Willoughby, 1971). Plumage coloration in desert birds,
thought to have evolved in response to predation, often closely
matches their substrate colour (Niethammer,1959), and does
not seem to play a significant role in thermoregulation. Desert
avifaunas are, to some extent, thought to be structured by pre-
dation and food resources. It has been suggested that raptors,
common desert inhabitants, have limited the diversity of other
species of birds to those adapted to the risks of living in an
exposed environment (Cook,1997). However, although a num-
ber of species of diurnal (Accipitridae, Falconidae and
Cathartidae), and nocturnal raptors (owls, Tytonidae and
Strigidae) occur in deserts, none of these are restricted to
deserts.
Using the Meigs (1953) definition of deserts, there are only
a few species of birds that occur primarily in desert habitats
(Serventy,1971). Many so-called desert species also occur out-
side deserts as well (Cowan,1997), and many birds in deserts
actually use “non-desert” habitats within the desert environ-
ment (Dean,2004). Avifaunas in Old World deserts are domi-
nated by coursers, sandgrouse, larks and chats, and by func-
tionally similar, but mostly unrelated, species in the deserts of
the New World (Maclean, 1996; Dean, 2004). This convergence
is due to the harsh environmental selective pressures to which
all birds in deserts have been exposed, and is manifested in sim-
ilar morphological, physiological and behavioural adaptations
in unrelated species (Maclean, 1996).
Our focus in this review is on the family Alaudidae, the larks,
and on the areas we know best, the Namib and Saudi Arabian
deserts. For each section we present an overview of desert birds
generally, and follow this with specific examples from the
Alaudidae. Lark species have penetrated deep into some of the
most hyper-arid regions on earth, occurring in sparsely vegetat-
ed stony deserts such as the Fezzan and Tibesti in southern
Libya (Guichard, 1955), the Rub’al Khali in Saudi Arabia
(Williams & Tieleman, 2000) and the Gobi in central Asia
(Tourenq et al.,1996a, 1996b). Adaptations in the desert-
dwelling species include lower basal metabolic rates and field
metabolic rates than larks of mesic habitats (Williams, 2001;
Tieleman et al., 2002, 2004), and lower daily fluctuations in fat
reserves than similar sized species of mesic habitats (Shkedy &
Safriel, 1991).
Transactions of the Royal Society of South Africa80 Vol. 59 (2)
DESERT AVIFAUNAS
Desert avifaunas (Cowan, 1997) are almost as difficult to define
as the environments that they occupy (Noy Meir, 1985). Based
on rainfall, we consider that any bird species that spends most
of its life in areas that receive < 250 mm mean annual precipi-
tation (MAP) to be a bird of arid regions, but that only those
species that occur mainly in deserts (sensu Meigs, 1953) to be
desert species.
Of the > 9730 bird species in the world, about 2500 occur in
arid to semi-arid areas, and of these 190 species also occur in
deserts (Dean, 2004). However, only 44 species world-wide
apparently spend most of their lives in hyper-arid environments
(Dean, 2004). Many of these species also move into more veg-
etated areas adjacent to deserts at times, or remain in the desert
at oases and drainage lines, i.e. in functionally “non-desert”
habitats within the desert environment, including wooded
drainage lines, rift valleys or oases, with few species in bare
dunes or on un-vegetated gravel plains (Dean, 2004), so there
are, in fact, very few species of desert birds. Of the 44 species,
14 are larks (Table 1), the highest proportion of “desert” species
in any bird family. However, in the Namib, only one species, the
dune lark Calendulauda erythrochlamys, is a resident largely
restricted to the desert; all the other species of larks in this
region, although common in the desert, or mainly occurring
within the desert, are nomadic or locally nomadic and use adja-
cent and more mesic habitats at times. In Saudi Arabia, in the
hyper-arid Rub’al Khali region, two species, greater hoopoe-
lark Alaemon alaudipes, Dunn’s lark Eremalauda dunni and
desert lark Ammomanes deserti are present all year round (JBW
unpubl data), but all three species may be locally nomadic or
nomadic in other areas (Keith et al., 1992). At least three other
larks, black-crowned Eremopterix nigriceps and grey-backed
sparrowlarks E. verticalis, and singing lark Mirafra cantans, are
not desert species per se, but are common in deserts, but also
occur in a wide range of more mesic savanna habitats.
Table 1. Larks (Alaudidae) that occur in deserts in parts of
their distribution ranges (adapted from Dean 2004). Only the
desert areas in which the species occur are given; many of the
species also occur outside deserts.
Taxonomically, 97 families of the >149 families of birds are
represented in deserts and semi-deserts, with Apodidae,
Strigidae, Caprimulgidae, Columbidae, Accipitridae,
Falconidae, Corvidae, Hirundinidae, Passeridae and
Fringillidae fairly strongly represented in most of the arid zones
of the world. With one exception ( the Alaudidae) phylogeny
does not provide insight into whether a species will be a
“desert” species or not. Some families, Laniidae, with 20 of 31
species, Passeridae, with 142 of 391 species and Fringillidae,
with 226 of 1015 species, are relatively strongly represented in
the arid zones of the world, but not all are desert species.
Accipitridae is well represented in the Asian and African arid
zones, but less species rich in the Australian, North and South
American arid zones, but similarly not all are desert species.
The Alaudidae has the highest proportion of species that inhab-
it deserts: of 96 species in the family, 85 are associated with
semi-arid or desert habitats (Dean, 2004). The family is also pri-
marily Old World, mainly African, where 76 species occur
(Dickinson, 2003). It is characterized almost throughout the dis-
tribution range of the family by a number of fairly narrow-range
endemics, markedly so in Africa (Keith et al., 1992). Of the 31
species that occur in southern Africa, for example, 24 species
are endemic or near-endemic to the region (Hockey et al., in
press). Larks differ from other oscine passerines in lacking an
ossified syringeal pessulus and in having scutes on the posteri-
or and anterior surface of the tarsi, and the hind claw is long and
nearly straight in almost all species. Their affinities are obscure,
and larks are generally thought not to be closely related to other
passerines.
Numbers of species, and abundance of birds in deserts is
low; patterns of species richness is strongly related to rainfall
amount, increasing rapidly with increasing rainfall (Maclean,
1996; Dean, 2004). Sparse vegetation in deserts strongly influ-
ences not only bird species richness but also the size and diet of
resident birds. Species that use desert habitats with sparse veg-
etation are relatively few, are usually small (< 50 g) and have a
mixed diet of invertebrates and plant parts, including seeds.
Nests or nest sites are usually not entirely dependent on vegeta-
tion, so many species nest on the ground or in crevices in rocks
(Dean, 2004). Conversely, some species of desert and semi-
Species Desert
Mirafra cantillans singing lark Arabia
Mirafra somalica Somali long-billed lark Sahel
Mirafra ashi Ash’s lark Sahel
Calendulauda erythrochlamys dune lark Namib
Eremopterix signata chestnut-headed sparrowlark Sahel
E. grisea ashy-crowned sparrowlark Sahara, Sahel, Arabia, India
E. nigriceps black-crowned sparrowlark Sahara, Sahel, Arabia
E. verticalis grey-backed sparrowlark Namib, Kalahari
Ammomanes cincturus bar-tailed lark Sahara, Arabia, Afghanistan, Baluchistan
A. phoenicurus rufous-tailed lark India
A. deserti desert lark Sahara, Sahel, Arabia, Baluchistan
Ammomanopsis grayi Gray’s lark Namib
Alaemon alaudipes greater hoopoe-lark Sahara, Sahel, Arabia, Afghanistan, Pakistan
Ramphocoris clotbey thick-billed lark Sahara, Arabia
Melanocorypha calandra calandra lark Iranian deserts, Afghanistan
Calandrella somalica rufous short-toed lark Sahel
C. rufescens lesser short-toed lark Arabia, Afghanistan
Eremalauda dunni Dunn’s lark Sahara, Arabia
Eremophila bilopha Temminck’s lark Sahara, Arabia
desert habitats are exceptionally large, with the largest birds of
all in African (common ostrich Struthio camelus and Somali
ostrich S. molybdophanes), and Australian deserts (emu
Dromaius novaehollandiae).
The presence of trees along drainage lines creates a more
structured plant community, and considerably increases the
species richness in desert areas (Cody, 1985, 1999; Maclean,
1996). In general, desert avifaunas show increasing species
richness and increasing residency with increasing vegetation
height and complexity (Dean, 2004). For example, the bird
species richness was significantly higher in wadis than adjacent
sparsely vegetated gravel plains in northern Saudi Arabia (van
Heezik & Seddon, 1999), and rifts and river valleys in the west-
ern Sahara have a richer, more sedentary avifauna than the
desert plains (Smith, 1965).
PLUMAGE AND PREDATION
The pale plumage of desert birds prompted some to argue that
the colour of desert birds is adaptive, helping to reduce heat
gain from solar radiation (Serventy, 1971; Maclean, 1996).
However, plumage colour is now thought to be adaptive in
another sense, that of reducing predation (Niethammer, 1959;
Willoughby, 1969; Cook, 1997). Colour of desert birds is fre-
quently related to soil colours (Serventy, 1971; Maclean, 1996).
In the Namib, at least 19 of 21 species of birds in hyper-arid
areas have “desert coloration” (pale sandy browns and greys)
and “generalised cryptic coloration” (dull browns and greys,
streaked or mottled), and only two species are conspicuously
coloured (Willoughby, 1969). The reduction in heat gain from
pale plumage colours would appear to be small (Willoughby,
1969, but see Serventy, 1971). Desert birds are sometimes black
or have black bellies. In the males of one of the conspicuously
coloured species, mountain wheatear Oenanthe monticola,
there are a number of plumage morphs, varying from all grey to
black-and-white; females are invariably plain dark brownish
black with a white rump (Maclean, 1993). Similarly, in one of
the common species of the Namib, the grey-backed spar-
rowlark, both sexes have black bellies, but are cryptically
coloured when viewed from above. It is possible that black bel-
lies, or black plumages reduce the cost of maintaining body
temperatures during early mornings, when ambient tempera-
tures are low. Black, black-and-white and dark brown are cer-
tainly common plumage colours in the Old World deserts
(Louw, 1993; Snow & Perrins, 1998). Black plumage, as in the
Chihuahuan raven Corvus cryptoleucus, absorbs a large amount
of solar radiation, but very little of this heat actually reaches the
skin surface. Air trapped between feathers and skin provides an
insulating layer (Louw, 1993). Dark-coloured birds can save
metabolic energy at low temperatures if they can expose them-
selves to solar radiation, but need to alleviate heat stress at high
temperatures by using shade and behavioural thermoregulation
(perching on stones in the wind, retreating down rodent burrows
and so on) (Serventy, 1971; Williams et al., 1999). Black
colours in the plumage do not negate the theory that plumage
colours are cryptic in desert environments; black plumages,
together with behaviour, may help to create disruptive patterns
and eliminate shadows and thus effectively conceal the bird
(Serventy, 1971).
Matches between soil colours and plumage colours are well
developed in the larks. Six lark species occur in the hyper-arid
Namib, of which three have plumage matching substrate
colours, and three have generalised cryptic coloration
(Willoughby, 1969). Similarly, the desert lark Ammomanes
deserti of the North African, Arabian and Indian deserts has
plumage colours ranging from pale sandy to very dark browns,
matching local substrates (Meinertzhagen, 1951). However,
although many argue that non-cryptic coloration in desert larks,
or desert birds in general, has been selected against by preda-
tion, no study has demonstrated or compared differences in pre-
dation rates between cryptically and non-cryptically coloured
species in desert environments.
Species matched in colour with small areas of soil are partic-
ularly vulnerable to climate change, if predictions of the effects
of climate change are true. In southern Africa, increasing tem-
peratures and sharply decreasing rainfall in arid areas (Midgley et
al., 2001) are expected to impact some bird species (Simmons et
al. 2004). Adaptable, generalist species may be able to cope with
climate change, but desert species, such as red lark Calendulauda
burra on red sands, and Gray’s lark Ammomanopsis grayi on
whitish to grey gravels, if shifted away from present ranges may
be disadvantaged on other substrate colours.
SURVIVAL
Physiological adaptations
Energy
Discussions of physiological adaptation to desert environments
typically include a description of adjustments of small mam-
mals, particularly kangaroo rats Dipodomys spp and the con-
centrating ability of their kidneys, arthropods and their imper-
meable cuticle, and even some arboreal amphibians that secrete
integumentary lipids to reduce cutaneous water loss (CWL),
whereas birds have been traditionally thought to be “pre-adapt-
ed” to the desert, or to lack any physiological specialization to
these harsh environments (Maclean, 1996). As one recent
Animal Physiology text suggests, “birds occur in deserts, but if
they need to drink, they can fly to watering places at some dis-
tance”, implying little selection on desert birds for physiologi-
cal specialization” (Hill et al., 2004). These notions are prima-
rily based on the early work of Bartholomew and his colleagues
who examined water loss of 13 species from the deserts of
southwestern North America and from nearby more mesic
regions; they concluded that birds in deserts have not evolved
unique physiological specializations that distinguish them from
mesic counterparts (Bartholomew & Cade, 1963). These
authors lamented the paucity of data on Old World species and
thought that these populations might show more conspicuous
physiological adaptations to arid conditions than their ecologi-
cal equivalents in the New World because Old World deserts are
geologically much older.
Because primary productivity in deserts is low, and thus food
resources scarce, selection should favour phenotypes that have
a reduced energy expenditure in these environments (Williams
& Tieleman, 2000). Several reports, subsequent to the work of
Bartholomew & Cade (1963), have hypothesized that arid-zone
birds may have evolved a reduced basal metabolic rate (BMR)
(Dawson & Bennett, 1973; Withers & Williams, 1990).
Selective advantages attributed to reduced BMR include lower
overall energy demand, lower total evaporative water loss
(TEWL), and lower endogenous heat production which would
have to be dissipated in a warm environment, often by evapora-
tive means. Tieleman & Williams (2000) compared the BMR of
21 species of birds from deserts with that of 61 species from
more mesic areas. Based on conventional least squares regres-
Adaptations of birds for life in deserts with particular reference to Larks 2004 81
Transactions of the Royal Society of South Africa
sion, and on regressions of phylogenetic independent contrasts
(Felsenstein, 1985), the analysis showed that, in general, desert
birds had a BMR 17-25% lower than non-desert forms (Fig. 1).
These broad-scale comparisons have been criticized because
species differ not only in environment but also in phylogenetic
history, diet and behaviour (Leroi et al., 1994); comparisons of
closely related species from different environments may provide
greater insights into how selection has influenced physiological
adjustments to environments without the complications of dis-
similar historical backgrounds. An examination of the BMR of
12 species of larks (Table 2) along an aridity gradient that
extended from The Netherlands to the hyper-arid deserts of
Arabia indicated that BMR decreased as the environment
became more arid (Fig. 2), a result consistent with the idea that
natural selection has reduced BMR in arid environments
(Tieleman et al., 2002). Interspecific phenotype-environment
correlations can indicate either genetic differences brought
about by natural selection or phenotypically plastic responses to
environmental conditions. In a separate study, Tieleman et al.
(2003) showed that adjustments in BMR could not be attributed
82 Vol. 59 (2)
Tab le 2. Species, mass, basal metabolic rates (BMR) and total evaporative water loss (TEWL) of larks (Alaudidae) from arid to mesic envi-
ronments. Data taken from Tieleman et al. (2003).
Species n Mass (g) BMR ± SD (kJ d–1)TEWL ± SD (g day–1)
Chersomanes albofasciata spike-heeled lark 20 25.7 29.1±4.98 25.0±3.33
Eremopterix nigriceps black-crowned sparrowlark 6 15.2 16.5±1.10 15.2±1.34
Eremopterix verticalis grey-backed sparrowlark –15.1 15.1±1.31
Ammomanes deserti desert lark 6 21.5 20.1±2.46 21.5±1.60
Alaemon alaudipes greater hoopoe-lark 21 36.9 32.8±4.45 36.9±2.59
Melanocorypha calandra calandra lark 2 50.6 49.5±1.07 50.6±3.03
Calandrella brachydactyla greater short-toed lark 8 24.0 35.6±6.73
Calandrella rufescens lesser short-toed lark 27 23.6 31.6±3.08
Eremalauda dunni Dunn’s lark 22 20.9 24.7±2.61 20.5±1.69
Spizocorys starki Stark’s lark 15.6±1.31
Galerida cristata crested lark 6 31.2 32.2±2.30 31.2±2.44
Lullula arborea wood lark 20 25.6 49.4±9.96 25.5±2.41
Alauda arvensis sky lark 29 32 62.4±8.43 31.7±3.47
Eremophila alpestris horned lark 26.0 28.6 (0 SD) 26.0±2.08
Figure 2. Mass adjusted average basal metabolic rate (BMR) in 12
species of larks (see Table 2) as a function of environmental aridity
(data from Tieleman et al., 2003).
Figure 1.A basal metabolic rate (BMR) and 1.B. field metabolic
rate (FMR) as a function of body mass in desert and non-desert
species (data from Williams & Tieleman, 2000). PIC phylogenet-
ic independent contrasts.
to acclimation of adults to thermal environment, food availabil-
ity, or photoperiod. Hence, these physiological differences are
likely the result of genetic differences, although work is
required to investigate the role of developmental plasticity.
The finding that BMR, a laboratory measurement, is reduced
in desert birds gains further evolutionary significance if it trans-
lates to a lower overall energy expenditure in the field.
Evaluating the hypothesis that desert birds have reduced field
metabolic rate (FMR), when compared to non-desert forms (as
measured by doubly-labelled water), Tieleman & Williams
(2000) showed that FMR of desert species was 49% lower than
mesic species (Fig. 1). Again restricting their analyses to larks,
Tieleman et al. (2004) examined the FMR of adults feeding 5-
8 day old nestlings from arid and mesic environments and found
that mass-adjusted FMR of arid-zone species was 36-42%
lower than that of larks in mesic habitats.
There seems to be considerable support for the idea that nat-
ural selection has influenced the rate of living among desert
birds. Differences in metabolism of temperate zone birds have
been attributed to selection for the size of organs with high
metabolic intensity, such as the liver, kidney and heart, required
to maintain levels of energy expenditure during the period when
parents care for nestlings, the putative time of peak energy
demand (Daan et al., 1990). However, when variation in body
mass is taken into account, organ sizes do not differ among
larks from deserts and mesic regions (Tieleman et al., 2003).
The differences in FMR and BMR among these species may be
related to variation in tissue-specific metabolic rates, an avenue
for future research.
Water and water use
The hot conditions of many deserts coupled with the scarcity of
drinking water creates the potential for physiological problems
associated with dehydration; these difficulties are especially
acute for desert birds because they are mainly diurnal, non-fos-
sorial, and have the highest mass-specific evaporative water
losses of all terrestrial vertebrates. Williams (1996) collated
rates of total evaporative water loss (TEWL), the sum of cuta-
neous and respiratory water loss, for 102 species ranging in size
from hummingbirds (Trochilidae) to common ostriches. Using
both conventional least squares regressions and regressions
based on phylogenetic independent contrasts, his analyses
showed that arid forms had a lower TEWL than species from
mesic environments. In a study on 12 species of larks from arid,
semi-arid and mesic environments (Fig. 3), Tieleman et al.
(2003) confirmed that TEWL decreased along an aridity gradi-
ent and suggested that natural selection was the most likely
explanation for the lower TEWL in desert forms.
In order to gain insights into the workings of natural selection
in desert environments, it is desirable to know the mechanisms
responsible for the reduced TEWL in desert birds. Several candi-
dates have been proposed that might function to reduce their
TEWL; hyperthemia which increases the temperature gradient
between the body and environment, thus reducing the need for
evaporative cooling (Calder & King, 1974; Weathers, 1981), a
counter-current heat exchange system in the nasal passages that
lowers respiratory water loss (Schmidt-Nielsen et al., 1970), and
adjustment of the lipid structure of the skin to reduce cutaneous
water loss (Menon et al., 1996; Williams, 1996). The role of
hyperthemia and of counter-current water recovery in the nasal
turbinates in reducing TEWL has been explored (Tieleman &
Williams, 1999), but these factors could not account for differ-
ences in TEWL between desert and non-desert species.
Figure 3. Mass adjusted average total evaporative water loss
(TEWL) in 12 species of larks (see Table 2) as a function of environ-
mental aridity (data from Tieleman et al., 2003).
Desert birds could reduce their TEWL by decreasing the rate of
water loss through their skin compared to more mesic species
(Williams, 1996; Williams & Tieleman, 2000). Although early
investigators surmised that most evaporative cooling took place
in the respiratory passages (Mount, 1979), later work showed
that CWL is an important avenue of water loss, at least at ambi-
ent temperatures below body temperature (Bernstein, 1969;
Webster & Bernstein, 1987; Wolf & Walsberg, 1996). A study on
CWL and respiratory (RWL) as a function of Tain greater
hoopoe-larks and Dunn’s larks from the Arabian desert, and sky
larks and wood larks from temperate grasslands in The
Netherlands showed that the contribution of CWL to TEWL in
larks ranged from 50-70% at moderate Tas, but at high Tas, RWL
dominated water loss (Tieleman & Williams, 2002a). Surface
specific CWL at 25ºC was 29% lower in arid-zone species than
in mesic larks suggesting that a reduction in CWL was a primary
determinant of the lower TEWL in desert birds. When acclimat-
ed to different Tas for 3 weeks, 15ºC-acclimated greater hoopoe-
larks increased CWL by 22% compared with 35ºC- acclimated
birds, but other species of desert birds or temperate-zone larks
did not change CWL. However, even with the increase in CWL,
rates did not equal those in mesic species. This study is consis-
tent with the idea that larks from deserts have a reduced CWL at
moderate and low Tas, but provided no support for the notion
that at high Ta larks from arid regions rely more on CWL than
larks from mesic environments. Interspecific differences in
CWL could not be attributed to acclimatory responses to envi-
ronmental temperature and were most likely the result of genet-
ic differences due to natural selection.
If water loss through the skin is reduced in desert birds, then
we need to understand the possible mechanisms that would pro-
duce this result. The skin of birds is composed of a thin outer
non-vascular epidermis and a thicker inner vascularized dermis
(Dyck, 1985). One component of the epidermis is an outer
cornified layer of non-living cells embedded in a lipid matrix
called the stratum corneum, the layer that forms the barrier to
water vapour diffusion from the animal to environment (Blank
et al., 1984; Elias, 1981; Bouwstra, 1997). Evidence to support
the idea that the stratum corneum forms the barrier to water
vapour diffusion comes from work on mammals that showed
Adaptations of birds for life in deserts with particular reference to Larks 2004 83
Transactions of the Royal Society of South Africa
that topical applications of organic solvents increased water
vapour loss (Scheuplein & Blank, 1971), that site-specific lipid
content of the stratum corneum correlated with barrier function
(Elias, 1981), that water vapour diffusion directly correlated
with removal of lipids from the stratum corneum (Grubauer et
al., 1989), and that replenishment of lipids of the stratum
corneum, from which they had been removed, restored barrier
function (Elias & Menon, 1991). Working on non-domestic
birds, Haugen et al. (2003) examined the relationship between
lipids of the stratum corneum and CWL of species of larks
along a temperature-moisture gradient. Results indicated that
free-fatty acids, cholesterol, and ceramides, large lipid mole-
cules containing a sphingoid base linked to a fatty acid via an
amide bond, were the major constituents of lipids of the stratum
corneum, just as in mammals, for all larks examined. CWL was
reduced in larks inhabiting deserts, but data did not support the
hypothesis that birds from deserts had larger quantities of lipids
per unit area of skin. Instead, they found that larks from desert
environments had a higher proportion of ceramides, and a
smaller proportion of free-fatty acids in their stratum corneum,
an adjustment that apparently reduced CWL. Subtle changes in
the ratios of lipid classes can apparently alter the movement of
water vapour through the skin. These authors hypothesized that
desert birds have a higher proportion of ceramides in their SC
and lower proportions of free-fatty acids because this combina-
tion allows the lipid lamellae to exist in a more highly ordered
crystalline phase and consequently creates a tighter barrier to
water-vapour diffusion.
THE RELATIONSHIP BETWEEN LIFE HISTORY
AND PHYSIOLOGY
Assuming that life-history trade-offs in terms of parental effort
are the result of energy allocation (Drent & Daan, 1980; Bryant,
1988), the proportion of energy devoted to reproduction versus
that spent on growth and maintenance can be measured
(Reznick, 1985). Tieleman et al. (2004) used parental energy
expenditure and water flux in the field as proxy for parental
effort, relating these variables to BMR and TEWL, and investi-
gated variation in clutch size among larks along an aridity gra-
dient as an independent measure of parental effort. Their results
show that, in terms of energy expended, parental effort, clutch
size, number of clutches and nestling growth rates decreased
with increasing aridity (Fig. 4). Parent-offspring energy budgets
for desert larks were 261 kJ/day for greater hoopoe-lark and 164
kJ/day for Dunn’s lark, but significantly higher for larks in
mesic habitats, equal to 388 kJ/day for sky lark and 347 kJ/day
for wood lark (Tieleman et al., 2004). Taking into account mass
differences between desert and mesic larks, desert larks used
27% less energy to raise a brood, and used 28-51% less water
per gram mass than larks in mesic habitats (Tieleman et al.,
2004). Nestling survival in desert larks was lower; with
decreased parental effort and smaller clutch sizes, single broods
may have a lower fitness value for desert larks, suggesting that
the probability of adult survival is higher in this group
(Tieleman et al., 2004).
Data on comparative rates of survival between arid zone
larks and larks of more mesic habitats are scarce. The survival
of dune larks in the Namib is high; birds ringed as adults have
been recaptured almost 6 years later (n 3, Williams, 1992)
and another adult, ringed by JBW was recaptured at > 7 years
old (Mark Boorman, unpublished data). The only other data
from southern Africa are for sabota larks Calendulauda sabota,
a species that occurs in both desert and relatively mesic wood-
lands. For six sabota larks, ringed as nestlings in dry woodlands
and recovered at the same site, the oldest was aged 4.5 years,
one was aged 2.2 years and the remainder at just over one year
(SAFRING unpublished data).
Survival of sky larks in Britain was similar to the survival of
sabota larks in southern Africa; 88% of 201 ringed sky larks
were recovered within five years of ringing, although some sur-
vived to 10 years of age (Dougall, 1996). No comparative data
are available for survival of larks in arid zones in the Northern
Hemisphere.
Parental effort and energy expenditure in desert larks may
also vary in a single species according to environmental condi-
tions. Lloyd (1999) showed that mean clutch sizes in spike-
heeled larks, grey-backed sparrowlarks and black-eared spar-
rowlarks Eremopterix australis increased after a substantial
rainfall in arid Bushmanland in South Africa (Table 3). The low
84 Vol. 59 (2)
Figure 4. Growth constant (K),
clutch size, number of clutches and
daily survival of six species of larks
(black-crowned sparrowlark, bar-
tailed desert lark, greater hoopoe-
lark, Dunn’s lark, wood lark, sky
lark) (data from Tieleman et al.,
2004). Aridity index is from most
arid (1.5) to mesic (3.5).
Adaptations of birds for life in deserts with particular reference to Larks 2004 85
amount of rain during 1993-1995 was insufficient to stimulate
breeding in the mainly granivorous sparrowlarks, but was suffi-
cient for breeding in the mainly insectivorous spike-heeled
larks.
FOOD
Food resources in deserts are mainly seeds, some invertebrates
and some vertebrates; there are very few juicy fruits offered by
desert plants, so frugivory is not an option in most deserts with
the exception of deserts of the New World where fruits of cac-
tus are seasonally abundant. Seed-eating birds are common in
deserts, feeding on fresh seed production or on wind-blown
seeds (Maclean, 1996; Cook, 1997; Dean, 2004). Resources in
deserts are patchy in space and time, but seed crops and associ-
ated invertebrates may be large and concentrated in fairly small
areas, leading to large numbers of birds that feed on them, and
this in turn attracts predators that feed on the birds (Dean,
2004). It follows that birds that feed on plants that grow fast,
flower and produce large quantities of seeds after irregular rain-
fall events, must be nomadic to some extent (Dean, 2004).
The larks tend to be generalists, feeding on seeds and inverte-
brates (Willoughby, 1971; Keith et al., 1992; Hockey et al., in
press), and those species that are locally nomadic, or nomadic
over larger distances (see Movements below) are more granivo-
rous than species that are relatively sedentary (Dean, 2004).
Willoughby (1971) examined stomach contents of six species of
larks, three sedentary, or relatively sedentary, and three nomadic
species in the Namib. All ate seeds and insects, and all had vary-
ing amounts of green vegetation in their stomachs (usually basal
nodes of grasses, but may also have been stomach contents from
grasshoppers that had been eaten). Among the residents, only
sabota larks ate more seeds (60%) than insects, whereas dune
larks and spike-heeled larks had 68% and 84% insects respec-
tively in their stomachs. For nomadic species, however, seeds
were a major part of the food and amounts in stomachs were from
56% seeds in Gray’s larks, 77% seeds in Stark’s larks and 91% of
the food in grey-backed sparrowlarks (Willoughby, 1971).
Greater hoopoe-larks in Arabia feed on a wide range of ani-
mal material, including termites (JBW unpubl data) and small
lizards (Keith et al., 1992). Snails, ant-lions (Neuroptera) and
hard seeds all seem important in their diet (Keith et al., 1992).
Other larks, such as thick-billed larks Ramphocoris clotbey,
desert larks and Dunn’s larks in Arabian deserts similarly feed
on seeds and invertebrates, occasionally on small lizards (Keith
et al., 1992), but relative proportions of plant and animal foods
are not known.
One other food item is frequent in the diet of desert larks,
may be common to all desert larks, and quite likely common to
all larks. Ants (Formicidae) are eaten by all species, including
all the largely granivorous sparrowlarks for which detailed
stomach contents data are known (Keith et al., 1992).
BEHAVIOURAL ADAPTATIONS
Thermoregulation
Activity patterns to reduce heat gain or water loss in desert birds
fall into four broad categories: (1) minimizing activity, (2) using
shade to reduce absorption of solar radiation, (3) repeated
bathing if water available, and (4) soaring to reduce radiation
and ambient temperature (Maclean, 1996). Concentrating activ-
ity into the beginning and end of the day, rather than the hot
period in the middle of the day, serves to reduce heat gain. In
Saudi Arabia, greater hoopoe-larks are inactive during the mid-
dle of the day, whereas larks in more mesic habitats forage
throughout the day (Tieleman & Williams, 2002b). Similarly,
many bird species, not only larks, in the Namib remain rela-
tively inactive during the middle of the day, spending the hot
hours in shade (Willoughby & Cade, 1967). An example is the
dune lark, that roost overnight in clumps of a reedy desert grass
Stipagrostis salbulicola and begins intensive foraging immedi-
ately after emerging from the roost, at about 20 minutes after
sunrise (Cox, 1983). During late winter, the birds foraged until
late morning, when the ambient temperature was about 25º C
and sand surface temperature had reached about 45º C, there-
after entering clumps of the grass for a period of inactivity that
lasted until ca 16h00 when they again began foraging (Cox,
1983). However, in summer, with soil surface temperatures
reaching 62º C, the larks continued foraging without a break,
resting between foraging sorties in S. salbulicola clumps
(Safriel, 1990). Similarly, Gray’s larks and spike-heeled larks
forage out in the open throughout the day (Willoughby, 1971).
Between foraging bouts in the hot parts of the day, Gray’s larks
lose heat by perching on stones or twigs a few cm above the
sand surface, facing into the prevailing westerly winds, and
holding their folded wings away from the body to expose thin-
ly feathered sides (Willoughby, 1971). Both Gray’s larks and
spike-heeled larks forage around the burrow entrances of
ground squirrels Xerus inauris and other rodent burrows, and
use the burrows as thermal refugia (Willoughby, 1971). There
are no hard data on diurnal activity patterns for other lark
species in the Namib; Willoughby (1971) implies that both
Stark’s larks and grey-backed sparrowlarks are inactive during
the middle of the day, but does not give times when the birds are
active. Stark’s larks lose heat, or more correctly avoid gaining
heat, by crouching in the shade of stones, overhanging rocks or
vegetation (Willoughby, 1971). Similarly, grey-backed spar-
rowlarks avoid heat gain by resting in the shade of small plants.
Maclean (1996) notes that tractrac chats Cercomela tractrac in
the Namib use the same behaviour as Gray’s lark to lose heat,
perching a few cm above the ground and facing into the cool sea
breeze.
The use of burrows as thermal refuges in other desert
ecosystems has been reported by Williams et al. (1999). Dunn’s
Tab le 3. Clutch sizes ± SD (n) of arid zone larks in South Africa after different sized rainfall events. Data from Lloyd (1999). Rainfall was:
1993-1995 no rainfall event > 25 mm, Sep-Nov 1996 54 mm in July, Nov-Dec 1996 78 mm in November.
Species 1993-1995 Sep-Nov 1996 Nov-Dec 1996
Chersomanes albofasciata spike-heeled lark 2.0 ± 0 (13) 2.83 ± 0.39 (23) 3.3 ± 0.78 (10)
Eremopterix verticalis grey-backed sparrowlark 2.37 ± 0.57 (121) 3.07 ± 0.46 (124)
E. australis black-eared sparrowlark 2.51 ± 0.53 (69) 2.93 ± 0.33 (44)
Transactions of the Royal Society of South Africa
larks, bar-tailed desert larks, black-crowned sparrowlarks and
greater hoopoe-larks used burrows of the large herbivorous
lizard Uromastyx aegypticus as thermal refugia during hot sum-
mer days in the Arabian desert. Soil surface temperatures in the
area exceeded 60º C on most days, whereas burrow substrate
temperature was about 41º C during midday (Williams et al.,
1999). For one species, the greater hoopoe-lark, there are sig-
nificant advantages in using burrows; the birds can potentially
reduce their water loss by as much as 81% by sheltering in the
lizard burrows during the hottest periods of the summer day
(Williams et al., 1999).
Soaring, as a means of losing heat, does not apply to larks.
For large species, such as vultures and eagles, soaring is an eff i-
cient way of losing heat. Air temperature decreases at the rate of
ca 3º C for every 300 m in elevation (Cook, 1997), so if a large
bird is soaring at 2400 m it would be experiencing ambient tem-
perature well within its thermoneutrality zone.
ANTI-PREDATOR BEHAVIOUR IN DESERT LARKS
Although predation is thought to structure avifaunas in deserts,
there are few actual observations of predation on birds in
deserts. Avian desert raptors tend to be “generalists”, taking
whatever prey they can get (Cook, 1997) and it is likely that this
would also apply to small predatory mammals. However, the
sooty falcon in the Sahara Desert specialises on passage
migrants, mainly small warblers (Brown et al., 1982), and in the
Namib, the red-necked falcon Falco chicquera is fairly spe-
cialised, commonly taking Gray’s larks and grey-backed spar-
rowlarks (Jensen, 1972) and Stark’s larks (WRJD pers obs).
Maclean (1970a) recorded a red-necked falcon (once) feeding
on an adult spike-heeled lark in the Kalahari, and suggested that
other small birds of prey may be included as lark predators.
More generalised lanner falcons F. biarmicus in the Namib prey
on Gray’s larks, grey-backed sparrowlarks and Stark’s larks but
also take sandgrouse (Pteroclidae) and doves (Columbidae)
(WRJD pers obs).
Adult dune larks are wary when foraging, and Safriel (1990)
notes that when the birds foraging at midday spotted an insect,
they emerged from their thermal refuge and “moved their heads
as if inspecting the sky for predators” before darting forward to
seize the prey. Terrestrial predators are likely to be as important
as aerial predators; Maclean (1970a) notes that bat-eared foxes
Otocyon megalotis are probably the commonest nest predator in
the Kalahari, and that Cape foxes Vulpes chama may also
account for some predation. Both these mammals are also like-
ly to prey on adult larks. Snakes, particularly Cape cobras Naja
nivea, are potential predators of both adult and nestling larks.
Predation on black-crowned sparrowlarks was thought to be
“exceedingly important” in shaping adult breeding bird behav-
iour and distraction displays (Morgan & Palfery, 1987). A list of
potential predators given by Morgan & Palfery (1987) included
large lizards, snakes, small mammals and birds. Shkedy &
Safriel (1992) considered that both aerial and terrestrial preda-
tors important in the Negev Desert, and noted that crested larks
were more vigilant than desert larks and this behavioural differ-
ence potentially reduced predation rates on nestlings.
Falcons in the Namib and Kalahari wait at waterholes, taking
the prey species as they arrive to drink or as they depart after
drinking. Under these circumstances there are few tactics that
the birds can use to avoid predation. The sandgrouse, doves and
larks arrive and depart in flocks, but this only reduces the prob-
ability of an individual becoming prey (see Hamilton, 1971).
Siegfried & Underhill (1975) demonstrated experimentally that
flock-feeding or flock-drinking does increase the probability of
individual laughing doves Streptopelia senegalensis feeding or
drinking optimally, and of detecting and avoiding a predator.
Although Willoughby (1971) states that burrows serve spike-
heeled larks and Gray’s larks as (inter alia) predator-escape tun-
nels, his observations were based on the behaviour of wounded
birds. No study has demonstrated that birds consistently dive
down burrows when a raptor appears.
MOVEMENTS AND NOMADISM IN DESERT
LARKS
Resources in deserts are patchy in time and space (Noy-Meir,
1985). Animal communities similarly vary in abundance and
local distribution (Polis, 1991), and most desert bird species are
nomadic at scales ranging from locally moving around in the
landscape to movements over large distances (Dean, 2004).
Nomadism, and local movements by birds in deserts are an
adaptation to the variability, and not the severity, of desert
ecosystems (Rowley, 1974).
It is this variability, rather than the lack of rain, that strongly
influences whether or not bird species are nomadic. Rainfall in
arid ecosystems is both low and variable, and has a high co-effi-
cient of variation (CV) (Le Houérou, 1988). Rainfall in arid and
semi-arid ecosystems in Africa (Tyson, 1986; Seleshi &
Demaree, 1995; Plisnier et al., 2000; Holmgren et al., 2001),
South America (e.g. Lima et al.,1999) and Australia (e.g.
Nicholls, 1991) is also greatly affected by El Niño events, lead-
ing to prolonged droughts, and La Niña events, usually with
extensive wet periods and concomitant effects on ecosystem
functioning (Holmgren et al.,2001). Fewer bird species occur
in areas where the rainfall is both low and highly variable
(Dean, 2004).
There are exceptions to the general rule that birds must move
around in order to survive in deserts. The dune lark is resident
and sedentary in areas that are fairly barren, hyper-arid and sub-
ject to large inter-annual variability in rainfall (Safriel, 1990).
Nevertheless, the birds remain within small circumscribed areas,
and birds ringed initially as adults have been recaptured at the
sites where they were ringed up to five years later (Williams,
1992). In contrast, Gray’s larks wander over “large expanses” of
the desert and did not show any tendency to remain within a cir-
cumscribed range (Willoughby, 1971). Stark’s lark and grey-
backed sparrowlark in the same area were more nomadic,
appearing in very large flocks when grasses were beginning to
grow (Willoughby, 1971). Grey-backed sparrowlarks, in fact,
wandered further and more freely than Stark’s larks and were
observed in the most barren parts of the Namib sand sea
(Willoughby, 1971). Similarly, Maclean (1970b) recorded large
fluctuations in numbers of Stark’s larks, pink-billed larks
Spizocorys conirostris, grey-backed and black-eared spar-
rowlarks in the Kalahari. Other larks in the area, including east-
ern clapper larks Mirafra fasciolata,fawn-coloured larks
Calendulauda africanoides, sabota larks and spike-heeled larks,
were apparently more sedentary (Maclean, 1970b). In
Bushmanland, in the Northern Cape, South Africa, red larks
Calendulauda burra are resident and largely sedentary on bare
to thinly vegetated red sand dunes in an area that receives <100
mm mean annual precipitation (Dean et al., 1991).
Black-crowned sparrowlarks are said to be migratory in
Saudi Arabia (Morgan & Palfery, 1987), but the movements of
the flocks and their appearances appear to be somewhat
86 Vol. 59 (2)
Adaptations of birds for life in deserts with particular reference to Larks 2004 87
nomadic. According to Meinertzhagen (1951), thick-billed
larks Ramphocorys clotbey are nomadic throughout their range
in the Sahara and Arabia, and the desert lark, bar-tailed desert
lark and Dunn’s lark are nomadic in the Sahara, Arabian and
Iranian deserts. A list of lark species thought to be nomadic in
the warm deserts of Arabia is given by Dean (2004) (Table 4).
REPRODUCTION
Displays in desert larks
A feature of the nomadic Gray’s Lark of the Namib is that the
flight song – the distinctive full or advertising song in larks – is
performed in the dark. The birds give a burst of singing about
half an hour after sunset, and then a prolonged session of
singing for about two hours before dawn and ending at sunrise
(Willoughby, 1971). Dune larks, Stark’s larks and grey-backed
sparrowlarks all display during the day. Dune larks deliver their
advertising song from perches or in the air, but Stark’s larks and
grey-backed sparrowlarks most frequently sing in the air, and
usually simultaneously in large numbers (Willoughby, 1971).
The flight song in Stark’s larks is loud and strong, delivered by
individual birds for several minutes at a time.
It is tempting to suggest that the nocturnal song and display
of Gray’s lark is to lower the probability of diurnal predation;
birds in display are probably less vigilant than at other times.
Gray’s Lark is found in low densities in the Namib. Stark’s lark
and grey-backed sparrowlark occur in thousands, and the prob-
ability of an individual being taken by a predator would be
lower, so the birds can enjoy displaying during the day.
However, dune larks display during the day, and are probably
the least abundant bird in the Namib, suggesting that the timing
of displays is not modified by the risk of predation.
Furthermore, monotonous larks Mirafra passerina that occur in
some high rainfall years on the edge of the Namib in large num-
bers display during the day and night, so there is little support
for the nocturnal avoidance of predation theory.
Pair-bonds and territories are known for some of the resident
larks; in dune larks pair-bonds seem to be fairly long-term, and
territories are held throughout the year. Dune lark males
increase the frequency of aerial displays when females are
building nests (Boyer, 1988). In greater hoopoe-larks in Saudi
Arabia, males hold territories, with pair-bonds lasting the entire
year, although some females will switch male partners during
the winter period (JB Williams unpubl data). Pair-bonds in the
nomadic larks appear to be transient, but sparrowlarks, for
example, are almost always seen in pairs, suggesting long-term
pair-bonds. Courtship displays in the nomadic larks vary from
intricate aerial manouvres with complicated songs to perch dis-
plays with monotonous songs, to quite simple ground displays
with simple calls (Willoughby, 1971, Keith et al., 1992).
Cues for breeding in desert larks
Breeding in desert birds is stimulated by rainfall (Maclean,
1970a; Immelmann, 1970, 1971; Willoughby, 1971; Rowley,
1990), including larks (Lloyd, 2004) but what is uncertain are the
visual cues that all desert birds (particularly nomads) need to
indicate where resources are abundant, and where it may be a
suitable area to which to move and nest. It is very likely the pres-
ence of green vegetation that provides some of the stimulus to
trigger breeding, and not only abundant food. The presence of
suitable nest material may also provide some stimulus for breed-
ing, but it is most likely a combination of the availability of sev-
eral resources that trigger breeding. One resource in abundance
will not trigger breeding; in an experiment in Saudi Arabia in a
“drought year”, six pairs of greater hoopoe-larks were provided
with food, including mealworms, and water for over one month
in the field. Only one pair attempted to breed, building a nest that
they subsequently abandoned (JBW unpubl. data).
Movement by the birds towards rainfall events will not
ensure that there are resources immediately available for repro-
duction because there is a lag between rainfall and seed pro-
duction, and rainfall and the response of potential insect prey
for the birds. Fairly strong evidence for cues for when to breed
comes from nomadic larks in the southern African arid and
semi-arid ecosystems (Dean, 2004). Black-eared sparrowlark,
grey-backed sparrowlark, Gray’s lark, pink-billed lark, Sclater’s
lark Spizocorys sclateri and Stark’s lark all use the awns of
desert grasses (Stipagrostis ciliata, S. obtusa and S. uniplumis)
as nest lining (Keith et al., 1992, Dean, 2004). The birds do not
start nesting until there is a good supply of suitable awns, and
this is visually evident from the air – the awns from these grass-
es are drifted by the wind into whitish heaps at the bases of
grass tufts, termite mounds, or any other obstruction to move-
ment. The pale heaps indicate that the grasses have set seed.
Conditions that lead to flowering and fruiting by the grasses
encourage increased surface activity in harvester termites
Hodotermes mossambicus (Coaton, 1958), an important food
for nestling larks (Keith et al., 1992).
Tab le 4. Nomadic larks (Alaudidae) species in deserts of Saudi Arabia and the United Arab Emirates. Data from Bundy & Warr (1980),
Clarke (1980), Phillips (1982), Rahmani et al. (1994), van Heezik & Seddon (1999), adapted from Dean (2004)
Species
Mirafra cantillans singing lark
Eremopterix nigriceps black-crowned sparrowlark
Ammomanes cincturus bar-tailed lark
A. phoenicurus rufous-tailed lark
A. deserti desert lark
Alaemon alaudipes greater hoopoe-lark
Ramphocoris clotbey thick-billed lark
Melanocorypha calandra calandra lark
Calandrella rufescens lesser short-toed lark
Eremalauda dunni Dunn’s lark
Transactions of the Royal Society of South Africa
NESTS AND NEST SITES
Many species of desert birds nest on the ground. In the larks, all
species nest on the ground in sites that vary from sheltered
places next to grass tufts or shrubs to open, exposed sites in
gravel patches (Keith et al., 1992). Resident larks in the Namib
place their nests against shrubs or on the sides of small hum-
mocks of sand (Willoughby, 1971; Boyer, 1988), but locally
nomadic and nomadic larks generally nest in rather more open
sites, but frequently against a stone or small plant (Maclean,
1970a; Willoughby, 1971). Nest sites in all larks in the Sahara
and Arabian deserts are similar to those in the Namib, usually
placed at the base of a grass tuft or shrub (Keith et al., 1992;
Snow & Perrins, 1998). Lark nests are unremarkable, except for
Gray’s lark that builds a thick-walled cup in a stone-lined hol-
low in the ground, thought to provide insulation from the sur-
rounding hot substrate. The other feature common to all lark
nests in all desert ecosystems is a rampart of small stones
almost invariably placed around the nest, the function of which
is unknown. It is evidently important in the context of nesting in
the desert; larks in mesic habitats only rarely place stones
around the nest rim (Snow & Perrins, 1998).
CLUTCH SIZES
Larks in deserts have smaller clutch sizes than larks in more
mesic habitats (Shkedy & Safriel, 1992; Maclean, 1993;
Tieleman et al., 2004) but average clutch sizes do not differ sig-
nificantly between resident and nomadic larks in southern
African arid ecosystems (Dean, 2004). In one of the few studies
comparing clutch sizes in a single species in desert and more
mesic parts of its range, Herranz et al. (1994) found that Dupont’s
lark Chersophilus duponti had a mean clutch size of 3.6 (n 33)
in Spain, but 2.76 (n 75) in Algeria and Tunisia (Keith et al.,
1992). However, clutch size in several small granivorous nomadic
lark species is significantly smaller than similar-sized, locally
nomadic granivorous canaries and buntings (Dean, 2004). As
already noted, clutch sizes in desert birds, including larks, tend to
increase (in a single species)with increasing amounts of rainfall
in desert ecosystems in southern Africa (Lloyd, 1999).
Populations breeding in any one area when environmental condi-
tions are favourable (Maclean, 1970a), are usually fairly well syn-
chronised because there is only a brief pulse in food abundance
that follows rain. Net primary production in the Namib Desert
(Louw & Seely, 1982) and other arid areas in southern Africa is
low and mostly occurs immediately after rain (Rutherford, 1980;
Louw & Seely, 1982). The young larks are fed on insects, main-
ly harvester termites and grasshoppers (Keith et al., 1992).
Foraging effort in termites is related to grass growth, so their
availability, like the seeds, is directly related to rainfall. Grey-
backed sparrowlarks respond quickly to rain, and the duration of
their breeding season varies according to the amount of rain, the
smaller the rainfall event, the shorter the breeding season
(Maclean, 1970a, 1970b). Selective pressure would thus be
against an extended breeding season, and would favour a short,
efficient season. The combination of the rain, grass growth and
termite activity leads to a high degree of synchrony (Lloyd, 1999)
which in turn allows little time for the development of a large
clutch of eggs and the provisioning of a large brood of young
(Dean, 2004). Each egg represents an additional 24 hours spent at
the nest or in the breeding area (Williams, 1966).
Other environmental factors in deserts and intrinsic adapta-
tions in desert birds have also increased selective pressure for
the benefits of small clutch sizes. The combination of low food
availability and aridity, that select for decreased energy and
water conservation, high temperatures that limit daily activity,
and high nest predation, among an array of factors peculiar to
living in a desert environment, all select for small clutch sizes
(Tieleman et al., 2004).
NESTLINGS AND NESTLING CARE
There is a trend in southern African arid ecosystems for small
arid-zone granivorous passerines to have significantly shorter
nestling periods (14.2 ± 4.9 days) than mesic-zone passerines
(17.5 ± 3.7 days) (Siegfried & Brooke, 1989). All desert larks
have nestling periods that are shorter than larks of mesic habi-
tats (Keith et al., 1992), and in all species nestlings leave the
nest before they can fly (Keith et al., 1992; Maclean, 1993). In
the sparrowlarks (Eremopterix spp) and Stark’s lark
(Willoughby, 1971), the nestling period may be as short as
seven days. Parental care may be divided; Morgan & Palfery
(1987) suggested that more than two young cannot be cared for
satisfactorily by the adult black-crowned sparrowlarks in Saudi
Arabia. Once the young had been led from the nest, each was
under the care of one parent, and that there was no further con-
tact between the two halves of the brood after the first day out
of the nest. There are no comparative published data on post-
nestling parental care in other desert larks.
If each adult in a pair cares for only one nestling, this may
confer an advantage to the nestling in that it then has the undi-
vided care of one parent and, perhaps, a greater chance of sur-
vival. Selection would thus favour an optimal clutch size of two
eggs or less. Sclater’s lark, with a full clutch of one egg (Lloyd,
1997), has evolved to the point where time spent on egg-laying
and incubation nestling has been minimized, and parental care
of the chick has been maximized. In this respect, co-operative
breeding, or pairs with helpers, is known for at least two species
of southern African desert larks; spike-heeled (Steyn, 1988) and
Gray’s lark (Boix-Hinzen & Boorman, 2003; Desmasius, 2003)
have been recorded breeding with one extra-pair helper. This
would increase parental care in situations where food is limited.
BREEDING SUCCESS IN DESERT LARKS
There are few comparative data on the breeding success of larks
in arid regions. In one of the few studies the breeding success in
larks in the Arabian desert was generally low compared to larks
of mesic habitats (Shkedy & Safriel, 1992; Tieleman et al.,
2004). In semi-arid Spain, Dupont’s larks had low breeding suc-
cess, and most losses were at the nestling stage; of 119 eggs in
33 nests, 114 eggs hatched, but only 18 young left nests
(Herranz et al., 1994).
In southern Africa, the production of young of resident and
nomadic larks in the Kalahari is low (Table 4). There are very
little comparative data for larks of mesic habitats in this region;
in the only study for which there is a large sample,
Winterbottom & Wilson (1959) found that red-capped larks
Calandrella cinerea nesting in a high rainfall area near Cape
Town, South Africa, had a mean production of 1.5 young/nest
from a clutch of 2 eggs and 2.0 young/nest from a clutch of 3
eggs. This is much higher than similar-sized species nesting in
desert areas (Table 5).
In the Namib Desert almost half the losses of eggs of dune
larks was thought to be due to predation (Boyer, 1988) possibly
by small mammals or birds. Similarly, eggs and young of sev-
88 Vol. 59 (2)
Adaptations of birds for life in deserts with particular reference to Larks 2004 89
eral species of larks in the Kalahari are lost to mammalian pred-
ators (Maclean, 1970a).
Predation on black-crowned sparrowlark eggs and nestlings
in eastern Saudi Arabia was recorded but not directly observed
(Morgan & Palfery, 1987). A list of potential nest predators
given by Morgan & Palfery (1987) included large lizards,
snakes, small mammals and birds. Breeding success in that
species, however, seems high; of 10 nests that were observed
from egg to nestlings that left nests at eight days old, the aver-
age brood size was 1.8 young, but recruitment to the population,
following further losses of nestlings, was thought to be only
0.18-0.36 young/pair into the population (Morgan & Palfery,
1987). Slightly further west and north, Shkedy & Safriel (1992)
studied breeding success and predation of nests of desert and
crested larks near Sede-Boqer in the Negev Desert (Table 5).
Desert lark nests had lower survival of eggs and nestlings (0.15)
than crested lark eggs and nestlings (0.24), with predation
thought to be important in reducing broods in both species. In
the Negev, crested larks lost 63.2% of eggs to predators, where-
as in Germany the same species lost only 25.1% of eggs
(Shkedy & Safriel, 1992). Predators of eggs and nestlings in the
Negev included domestic cats Felis catus, foxes Vulpes spp.,
white storks Ciconia ciconia, brown-necked ravens Corvus rufi-
collis, great grey shrikes Lanius excubitor, stone curlews
Burhinus oedicnemus and transient raptors (Shkedy & Safriel,
1992). Crested larks were more vigilant than desert larks and
this behavioural difference potentially reduced predation rates.
CONCLUSIONS
Desert birds face two selective pressures, extreme heat and arid-
ity. Current evidence suggests that birds have evolved both
behavioural and physiological adjustments to the desert envi-
ronment, and that previous notions of “pre-adaption” were
based on insufficient data. Many desert birds differ from birds
of more mesic habitats, particularly in coloration and behaviour,
and less in diet, reproduction and moult. Behavioural adapta-
tions include nomadism, the use of shelters to avoid the heat of
the day and opportunistic breeding. Physiological adaptations
include economy of water use and lower energy expenditure.
The larks (Alaudidae) are a family of birds well-adapted for
desert environments. Adaptations in desert species include
decreasing field metabolic rate, basal metabolic rate and reduc-
tions of cutaneous water loss with increasing aridity (Tieleman
et al., 2002, 2003, 2004; Tieleman & Williams, 2002a), and
decreased parental effort, clutch size, number of clutches and
nestling growth rates with increasing aridity (Tieleman et al.,
2004). Other adaptations include behavioural responses to the
problem of losing heat, nocturnal displays, quick responses to
breeding after rainfall events, short nestling periods, brood divi-
sion, and in some species, co-operative breeding to maximise
parental care.
ACKNOWLEDGEMENTS
This paper was presented at the colloquium on “Adaptations in
desert fauna and flora” Victoria West, 29-31 August 2004. The
attendance by WRJD at the colloquium was partly funded by
the German Ministerium für Bildung und Wissenschaft
(BMBF) under project number 01LC0024 (BIOTA – southern
Africa), and by the National Research Foundation (South
Africa). Financial support for research on the Alaudidae by
JBW was made available by the National Science Foundation
(USA) (Project no. IBN-0212092).
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... Specifically, 83 species of larks breed on the continent (or adjacent islands such as Cape Verde), of which 63 are endemic to Sub-Saharan Africa (based on the taxonomy proposed by . Alaudidae is primarily an African family, and exhibits the highest proportion of species associated with semi-arid or desert habitats (Dean and Williams 2005). This family is also represented in Eurasia (42 species) and, to a lesser extent, in Australasia and the New World (one species each, both of which also occur in other regions). ...
... Larks constitute the only famiily in the large passerine superfamily Sylvioidea (sensu Fregin et al. 2012, Fjeldså et al. 2020) that is terrestrial, and one of only two that occur in open habitats, and as a group they inhabit practically every type of dry or semi-dry open habitat. Since this family has the highest proportion of bird species that inhabit deserts (~85% of larks are associated with semi-arid or desert habitats; Dean and Williams 2005), one might expect that the colonization of this novel ecological space, as well as other open habitats such as steppe grasslands, increased rates of speciation and phenotypic diversification. Yet, regarding the latter, larks do not exhibit a striking phenotypic disparity; most species exhibit a cryptic plumage that matches the soil colour (Donald et al. 2017) and show a moderately conservative body form, except for the shape of the beak, which ranges from short, heavy, and conical to elongated and thin (De Juana et al. 2004). ...
Article
Full-text available
Larks (Alaudidae) constitute one of the avian families best adapted to xeric environments, having colonized a wide suite of open habitats including deserts. Although their highest diversity is in Africa, larks occur on all non-polar continents. We tested whether larks exhibit exceptional and/or correlated shifts in the tempos of speciation and ecological trait diversification in the face of open ecological space. We employed a near-complete phylogeny and a morphological dataset including several recently recognized species. We found homogeneity in diversification dynamics across the family and evidence for a diversity- dependent slowdown in cladogenesis, which indicates that Alaudidae may approach their “ecological limit”. We did not observe an early-burst in phenotypic diversification, as would be expected in a “classic” adaptive radiation. Our findings suggest that larks’ morphology show a high level of evolutionary conservatism and overall lack ecomorphological convergence: ecological variables (diet and habitat) —which by contrast display a higher lability— explain little of shape/size variation except beak shape. Both adaptation to aridity and dietary transitions have evolved independently in multiple lineages across subfamilies. This study supports the idea that continental radiations in open habitats may reach an equilibrium faster in comparison with those in tropical forests, due to differences in ecological opportunities.
... Fig. 15.15 Greater Hoopoe-Lark (Alaemon alaudipes), a species highly adapted to life in Arabia, but which regularly forages and as seen here, collects nesting material, at anthropogenic sites. Photographer: Oscar Campbell Other adaptations to living in extreme aridity include generalist diets that may shift opportunistically, smaller clutch sizes, fewer clutches per breeding season and relatively slow rates of nestling growth, meaning that adults do not risk having to continually forage (Dean and Williams 2004). Nestling fitness is also reduced, and this implies higher adult survival and greater longevity. ...
... Data on breeding bird distribution and ecology was summarized by Aspinall (2010); Jennings (2010) eloquently and comprehensively puts the UAE's breeding avifauna into a regional context. Dean and Williams (2004) summarize bird adaptations to life in deserts, particularly larks, with examples from research undertaken in Saudi Arabia. ...
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The United Arab Emirates may be a small, arid country but is home to a surprisingly diverse and interesting range of birds that occur as breeders, migrants and winter visitors. The majority of bird species in the UAE occur as migrants and/or winter visitors; of those that breed the majority are cosmopolitan or Saharo-Sindian in biogeographic origin. The country’s avifauna can be broadly related to seven major habitats. To survive the extreme climatic conditions prevalent, birds show remarkable adaptations, both behavioural and physiological. Ornithology is an evolving science in the UAE and new discoveries continue to be made.
... Many desert ant species maintain mutualistic relationships with desert plants bearing extrafloral nectaries, including endemic cacti species globally threatened with extinction (Aranda-Rickert et al., 2014;Goettsch et al., 2015). Deserts are inhabited by a diversity of myrmecophagous animals, spanning desert birds with a high proportion of ants in their diet relative to temperate environments (Dean & Williams, 2004) to ant-specialist horned lizards (Phrynosoma) (Whitford & Bryant, 1979). Additionally, ant nests host a diversity of myrmecophiles (e.g., socially parasitic invaders) (summarized in Hölldobler & Kwapich, 2022). ...
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Land-use change from solar energy development may affect desert ecosystems and the soils, plants, and animals therein, yet our understanding of these interactions is nascent. With their ubiquity, criticality as ecosystem constituents, and sensitivity to environmental variation, ants may be useful study organisms for elucidating ecological effects of solar energy development in deserts. Our objectives were to disentangle the response of a desert ant community to solar energy development decisions and test the efficacy of ants as bioindicators at a solar power facility (392 MW) in the Mojave Desert, USA. We used pitfall traps to collect ants in treatments representing different solar energy development decisions , including variably intense site preparation practices: blading (i.e., bulldozing) and mowing, and establishment of undeveloped patches in solar fields, replicated across three power blocks comprising the facility and in undeveloped control sites surrounding the facility. We determined that ant abundance, species richness, Shannon Diversity Index, and functional richness were lower in bladed treatments than in all other treatments and controls. For most taxonomic and functional ant responses, we detected no difference between nonbladed treatments and controls; these results suggest that less intensive site preparation and increased spatial heterogeneity (i.e., undeveloped patches in solar fields) can reduce the negative effects of solar energy development on desert ants. However, our results indicate that ants may serve as useful bioindicators of the severity of anthropogenic disturbance from solar energy development in deserts, and indicator analysis signifies that solar energy infrastructure may negatively affect some species with high ecological value (e.g., harvester ants). Negative effects of solar energy development on ants can have significant implications for desert ecosystem function and integrity, but conservation-minded solar facility design and construction may lead to avoidance of "bottom-up" ecological ramifications of increased solar production during the renewable energy transition.
... Shrubs and trees were also used as thermal refugium to avoid heat stress during the hottest hours of the day, when the temperature of the ground surface may exceed 50 or even 60°C (Zwarts et al. 2023f). The need to avoid such heat is evident given the widespread habit among birds of the (semi-)arid zone to search for shade, or when there is no shade from trees to search for a termite mound, stick or branch to escape the heat of the ground surface (Williams & Tieleman 2001, Dean & Williams 2004, Manu & Cresswell 2013, Martin et al. 2015. ...
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This paper quantifies the density and the total number of granivorous and insectivorous ground-foraging birds, whether Afro-Palearctic migrants or Afro-tropical residents, in the transition zone between the arid Sahara and the humid Guinea zone. Situated between 17°W and 42°E and between 7°N and 22°N, this is an area covering 10 million km2. The study took place during the northern winter, between 20 November and 10 March (thus covering much of the long dry season) from 2011 up to and including 2019. Using a stratified random sampling regime, we counted birds at 1901 sites of 4.5 ha in area. We present background information about the study region, with maps showing variation in elevation, rainfall, woody cover, land use and human population density. The bird counts were converted into average densities for 43 bird species in 150 grid cells of 1° latitude × 1° longitude. The distribution of the various bird species was predominantly related to annual rainfall, but because woody cover increases with rainfall, species’ preferences for arid or more humid zones were partly influenced by an overall preference for open or more wooded landscapes. Bird species such as larks and Tawny Pipit Anthus campestris, even when rainfall was accounted for, selected comparatively open landscapes, whereas species feeding on the ground near trees or using them as perches (e.g. sparrows, finches, shrikes, Tree Pipit Anthus trivialis) preferred relatively more enclosed environments. To estimate total population size, the 150 grid cells were assembled into eleven rainfall categories (per 100 mm rainfall) and six longitudinal bands. To assess the reliability of these estimations, population sizes were calculated separately on the 1901 study sites split in two halves. The estimated population sizes were precise for migrants, especially for insectivores (7% deviation for the split-half estimates), but less precise for residents (22–28% deviation). Most ground-foraging birds were granivorous (at least in the dry season), their total number being estimated at 4000 million residents and 133 million migrants, residents being 30 times as abundant as migrants. Ground-foraging insectivores were less numerous, the total estimated being 920 million birds, of which 694 million were residents and 221 million migrants, the ratio residents/ migrants being an order of magnitude smaller than in granivores. The three most abundant granivorous residents were Red-cheeked Cordon-bleu Uraeginthus bengalus (467 million), Sudan Golden Sparrow Passer luteus (375 million birds) and Red-billed Quelea Quelea quelea (311 million). The Greater Short-toed Lark Calandrella brachydactyla (126 million) was the only common granivorous migrant. The most common insectivorous ground-foraging bird was a resident (Greater Blue-eared Starling Lamprotornis chalybaeus; 100 million), and more commonly encountered than all the ground-foraging insectivorous migrants such as Isabelline Wheatear Oenanthe isabellina (32 million), Northern Wheatear Oenanthe oenanthe (27 million) and Western Yellow Wagtail Motacilla flava (24 million) together.
... Due to his ornithological experience, birds represented a convenient "lens" for Richard to examine Karoo ecology, such as the positive diversity-productivity relationship at the dry end of the curve (Dean, 2000b), the connection of endemism to geological features, behavioural avoidance of heat and low metabolism and reduced evaporative water loss (Dean and Williams, 2004). The prevalence of low densities of resident bird species in low-productivity areas and dietary specialists that tend to forage in groups, opportunistic breeding, and the irruptive dynamics of nomadic species during brief times of high productivity were the subjects of Richard's PhD thesis (Dean, 1995), expanded in his book, Nomadic Desert Birds (Dean, 2004). ...
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In August 2022, the renowned ecologist and ornithologist, Dr William Richard John Dean (Richard), passed away aged 82. He had served on the Editorial Board of the Journal of Arid Environments for 20 years until 2021. Richard’s penchant for writing, both scientific (294 journal articles and book chapters, 6 books) and popular science, contributed significantly to what is known about arid regions globally and to the Karoo, an area covering a third of South Africa and Namibia, in particular. His encyclopaedic knowledge was legendary, aptly remarked by Timm Hoffman (pers. comm.): “Richard was the best naturalist I have ever met. Many of his publications reflect his broad integration of different influences on the environment, whether by birds, ants, aardvarks, plants, soil or seeds.” Roy Siegfried (pers. comm.) added that Richard was more than an avid naturalist, as he had such an astute questioning mind, the mark of a great scientist. Indeed, Richard’s contributions to the ecology of arid environments are globally relevant. Here, we pay tribute to Richard Dean by expounding some of his significant scientific contributions to arid land ecology, some of which were developed in synergy with his wife, Sue Milton (Fig. 1), while recognising that he was also a leading ornithologist in his own right. We start with an overview of his professional development and then unfold a few examples of his notable insights into the Karoo and other drylands and what these mean for understanding arid lands in general, concluding with a synopsis of his outlook.
... Arid-zone birds are very sensitive to increasing air temperatures and heatwaves, which could broadly influence their water balance, daily activity, or geographic distribution (Albright et al. 2017). Birds from deserts show several adaptations to inhabiting such habitats (Dean & Williams 2004), and also have reduced basal and field metabolic rates compared with species from mesic areas (Tieleman & Williams 2000). In such areas, freestanding water is especially important during hot days (Smit et al. 2019). ...
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Water is crucial for birds, especially during hot weather. However, the availability of water, and its use by birds in modern anthropogenic habitats, is far from understood, especially outside arid regions. Here, we analyze a large nationwide dataset collected in the temperate zone and present an overview of small water resources used by birds in urban and rural habitats in Poland. We investigated the proportion of birds using free-standing water, preferences for various water sources, and factors and threats influencing drinking and bathing behaviour. Birds using water resources are represented by various taxonomic and ecological groups. Species composition differed slightly due to environmental conditions in the vicinity of the water resource and the background species composition. In total 51 species were observed using water, representing 64% of the 80 species recorded in the vicinity. The probability of water usage was positively related to temperature, which further emphasizes the importance of water under future climate-warming scenarios. We show that small water resources, including those provided by people, were less likely to be used by birds than resources resembling natural waters (puddles, ponds, fountains). This novel finding may have particular importance for avian conservation planning, including appropriate behaviour for nature lovers (providing water sources and reducing stress to birds due to predation risk). Finally, we assessed potential threats to bathing and drinking birds, such as moving cars, risk of drowning, and the presence of predators. Any kind of surface water is currently beneficial for wild birds inhabiting human modified landscapes. During heatwaves and droughts access to water can be crucial for many birds. Unfortunately, such extreme events are predicted to become more frequent and more severe under climate change. Therefore, we would encourage further research in the use by birds of free-standing water, similar to the many studies of birdfeeders in winter, and to consider the maintenance of diverse sources of accessible water in environmental management.
... Our most surprising finding is the comparatively low T b max of arid-zone birds, despite them experiencing the highest T air maxima, greatest water scarcity, and thus strong selection for water conservation (27,46). Moreover, compared with species from the other two sites, arid-zone species had lower T b max, yet tolerated significantly higher T air values (Fig. 3). ...
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Physiological performance declines precipitously at high body temperature ( T b ), but little attention has been paid to adaptive variation in upper T b limits among endotherms. We hypothesized that avian maximum tolerable T b ( T b max ) has evolved in response to climate, with higher T b max in species exposed to high environmental heat loads or humidity-related constraints on evaporative heat dissipation. To test this hypothesis, we compared T b max and related variables among 53 bird species at multiple sites in South Africa with differing maximum air temperature ( T air ) and humidity using a phylogenetically informed comparative framework. Birds in humid, lowland habitats had comparatively high T b max (mean ± SD = 45.60 ± 0.58 °C) and low normothermic T b ( T b norm ), with a significantly greater capacity for hyperthermia ( T b max − T b norm gradient = 5.84 ± 0.77 °C) compared with birds occupying cool montane (4.97 ± 0.99 °C) or hot arid (4.11 ± 0.84 °C) climates. Unexpectedly, T b max was significantly lower among desert birds (44.65 ± 0.60 °C), a surprising result in light of the functional importance of hyperthermia for water conservation. Our data reveal a macrophysiological pattern and support recent arguments that endotherms have evolved thermal generalization versus specialization analogous to the continuum among ectothermic animals. Specifically, a combination of modest hyperthermia tolerance and efficient evaporative cooling in desert birds is indicative of thermal specialization, whereas greater hyperthermia tolerance and less efficient evaporative cooling among species in humid lowland habitats suggest thermal generalization.
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Animal coloration serves many biological functions and must therefore balance potentially competing selective pressures. For example, many animals have camouflage in which coloration matches the visual background that predators scan for prey. However, different colors reflect different amounts of solar radiation and may therefore have thermoregulatory implications as well. In this study, we examined geographic variation in dorsal patterning, coloration, and solar reflectance among horned larks (Eremophila alpestris) of the western United States. We found that plumage brightness was positively associated with soil granularity, aridity, and temperature. Plumage redness-both in terms of saturation (i.e., chroma) and hue-was positively associated with soil redness and temperature, while plumage patterning was positively associated with soil granularity. Together, these plumage-environment associations support both background matching and Gloger's rule, a widespread ecogeographic pattern in animal coloration. We also constructed thermoregulatory models that estimated cooling benefits provided by solar reflectance profiles of the dorsal plumage of each specimen based on the collection site. We found increased cooling benefits in hotter, more arid environments. Finally, cooling benefits were positively associated with residual brightness, such that individuals that were brighter than expected based on environmental conditions also had higher cooling benefits, suggesting a trade-off between camouflage and thermoregulation. Together, these data suggest that natural selection has balanced camouflage and thermoregulation in horned larks, and they illustrate how multiple competing evolutionary pressures may interact to shape geographic variation in adaptive phenotypes.
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Arid regions globally have suffered ecological damage as nomadic cultures have virtually all been displaced over the past 300 years by European colonists who, in attempts to create permanent settlements, exploited grazing, fauna and water unsustainably. The Karoo is one such arid region that covers about one third of South Africa. In common with arid regions globally, this sparsely-populated shrubland was historically used mainly for ranching, but is now becoming increasingly important for renewable energy generation (wind and sun), for adventure tourism, and potentially for mining. Current and future land users in the Karoo face challenges posed by damage caused by past land use, particularly overgrazing and impoundments. Land users of the future will probably experience additional challenges caused by the combined impacts of damaging land use and climate change. This review synthesizes the bodies of literature on the archaeology, ecology, land use history, environmental change and development planning. In doing so, we address the effects of historical land use on biodiversity and ecosystem goods and services in the region. We also identify avenues for future research, including mitigation and restoration actions required to make continued human occupation of the Karoo sustainable. Insights on the sustainability of the Karoo region are relevant when considering the socio-ecological futures of arid regions elsewhere.
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Previous studies have demonstrated that perturbations in barrier function stimulate epidermal lipid synthesis and that this increase can be prevented by occlusive membranes. These observations suggest that epidermal lipid synthesis might be related to barrier function and raised the question whether transcutaneous water flux might regulate epidermal lipogenesis. In the present study we first abrogated the barrier with acetone, and then compared the rate of repletion of stainable lipids, barrier recovery, and epidermal lipogenesis in animals covered with occlusive membranes or vapor-permeable membranes versus uncovered animals. Acetone treatment of hairless mice removed stainable neutral lipids from the stratum corneum, with repletion evident both biochemically and histochemically within 48 hr in uncovered animals. In contrast, when the animals were covered with an occlusive membrane, the usual return of stratum corneum lipids was aborted. Since application of vapor-permeable membranes allowed normal lipid repletion, occlusion alone is not responsible for the inhibition of lipid repletion. Acetone treatment also perturbed epidermal barrier function, which returned to normal in uncovered animals in parallel with the reappearance of stratum corneum lipid. However, when animals were covered with an occlusive membrane, barrier function did not recover normally. In contrast, occlusion with vapor-permeable membranes allowed barrier function to recover normally. Finally, whereas occlusive membranes prevented the characteristic increase in epidermal lipid synthesis that follows barrier perturbation, epidermal lipid synthesis was increased in animals covered with a vapor-permeable membrane. These results point to transepidermal water flux itself as the signal that regulates epidermal lipid synthesis, which is associated first with the redeposition of stratum corneum lipids and then the normalization of stratum corneum barrier function.
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This article presents a list of ornithological observations, with notes, made during autumn 1992 and summer 1993, principaly in south-west Mongolia. The autumn observations show how important this area is for migrating or breeding raptors, such as; Black Vulture Aegypius monachus. Lammergeier Gypaetus barbatus, Steppe Eagle Aquila nipalensis, Golden Eagle Aquila chrysaetos, Upland Buzzard Buteo hemilasius, Roughlegged Buzzard Buteo lagopus, Long-legged Buzzard Buteo rufinus, Saker Falco cherrug and Peregrine Falcoperegrinus. 10 species new to the Gobi National Park were recorded; Solitary Snipe Gallinago solitaria, Curlew Numenius arquata orientalis, Black-winged Stilt Himantopus himantopus, Kentish Plover Charadrius alexandrinus. Steppe Eagle Aquila nipalensis, Hen Harrier Circus cyaneus, Montagu's Harrier Circus pygargus, House Martin Delichon urbica, Père David's Snow Finch Montifringilla davidiana and Chestnut Bunting Emberiza rutila, and during the same period 5 species were new for south-west Mongolia; Baïkal Teal Anas formosa, Curlew Sandpiper Calidris ferruginea, Curlew Numenius arquata orientalis, Black-winged Stilt Himantopus himantopus and Chestnut Bunting Emberiza rutila. Observations made during summer 1994 confirmed the known breeding of Black-winged Stilt Himantopus himantopus as well as the importance of wetlands visited in this dry area for migrant birds. Observations made around the Mongolian capital, Ulan Bator, support those already published, mainly by German and Russian authors.
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It is over twenty years since Jared Diamond focused attention on the possible existence of assembly rules for communities. Since then there has been a proliferation of studies trying to promote, refute or test the idea that there are sets of constraints (rules) on community formation and maintenance (assembly). This timely volume brings together carefully selected contributions which examine the question of the existence and nature of assembly rules with some rigour and in some detail, using both theoretical and empirical approaches in a variety of systems. The result is a balanced treatment which encompasses a wide range of topics within ecology including competition and coexistence, conservation and biodiversity, niche theory, and biogeography. As such it provides much to interest a broad audience of ecologists, while also making an important contribution to the study of community ecology in particular.
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Galahs are the commonest of the cockatoos (Psittacidae: Cacatuinae), and one of the few native species that have thrived throughout European settlement in Australia. Previously confined to within flying distance of tree-lined water courses, galahs have now expanded into rangelands and have invaded the wheatlands where their consumption of grain and fouling activities made them pests. Following comments on the study area and methods of research, this monograph describes: the environment, food resources and pest status of this species; elements of behaviour; vocalizations; social behaviour; behaviour of the breeding pair; social organization and daily activity pattern; breeding biology (nest hollows, from eggs to hatching, and nestlings); and productivity and survival. Previous conflict between galah and man has virtually disappeared in Western Australia, and the main problem will be the need to plant or encourage in reserves and shelter belts trees capable of developing hollows when mature (an increasingly rare resource) in order to conserve this species. -P.J.Jarvis
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The amount and reliability of rainfall covary across South Africa from the arid northwest to the humid east. This is taken as a measure of environmental unpredictability to predictability. Dry, intermediate and wet regions are recognized on this basis. Volant terrestrial bird species breeding in South Africa were allocated to the region or regions in which they breed. Comparisons were usually made between species breeding only in the dry or dry and intermediate regions with those breeding only in the wet or wet and intermediate regions. Available data (mean female body mass, modal clutch size, mean incubation and fledging periods in days) were assembled to examine to what extent environmental unpredictability correlated with variation in these parameters, the expected variation being lower mass, larger clutch size and shorter incubation and fledging periods with increasing unpredictability. Mass averages lower but not by a significant amount. Clutch size does not differ at all. However, in insectivorous birds with breeding ranges extending from dry to wet regions there is some significant evidence for larger clutch sizes in the dry region. This does not hold for granivorous birds. Incubation and fledging periods are significantly shorter in passerine birds in the more unpredictable dry region but this does not hold in nonpasserine birds. The results are discussed and it is concluded that experimental work, for which birds are not well suited, is needed to test the effects of different environments on life-history parameters, rather than further correlation studies.
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Metabolic heat production (), evaporative heat loss (), and body temperature () were measured simultaneously in six species ranging in size from 12 to 125 g. These data, together with those of previous studies, indicate that is not maintained constant at ambient temperatures () below the upper critical temperature (), but increases linearly with above about 30 C. Consequently, increases within the thermoneutral zone while remains unchanged. Plotting within the thermoneutral zone as a function of indicates that metabolism is independent of (i.e., ) in birds smaller than about 100 g. Calculations of rates of heat exchange show that hyperthermia contributes little to the cost of avian temperature regulation in hot environments, while it significantly enhances water conservation. In all species increased linearly with increasing above . The slope of the line relating to above is an integrated measure of the cost of thermoregulation in hot environments, here designated as the coefficient of heat strain (). In 26 species of birds, (mW gl⁻¹C⁻¹) varies with size according to the relation log , where m is body mass in grams. At the size of the gradient was inversely related to body size, decreasing from 5.0 C in the 12-g blackthroated sparrow (Amphispiza bilineata) to 1.2 C in the 125-g Gambel's quail (Lophortyx gambelii). Consequently, small birds depend less on evaporative cooling at than large birds, a circumstance favoring water economy in small birds. All species were able to increase during heat exposure without increasing . The extent to which increased without a concomitant increase in was, however, inversely related to body size. This may reflect a greater capacity to alter the plumage vapor gradient in larger birds.