ArticlePDF Available

Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability


Abstract and Figures

Naef-Daenzer, L., Naef-Daenzer, B. and Nager, R. G. 2000. Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability. - J. Avian Biol. 31: 206-214. We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies present data on foraging decisions in captive Great Tits. Little is, however, known about prey selection in the field in relation to the food available and the consequences this has for the food delivery rate to nestlings. Since the foraging performance of the parents is one of the main determinants of fledging weight and juvenile survival, foraging behaviour is an important part of Great Tit reproduction. During the early breeding season up to 75% of the prey biomass delivered to the nestlings were spiders, which is in contrast with other studies. Only when caterpillars reached a size of 10-12 mg (approximately the average size of the spiders caught at that time) did the Great Tits change their preferences and 80-90% of the delivered prey masses were caterpillars, as reported by other authors. This 'switching' between prey occurred within a few days. It was not related to the changes in abundance but to size of caterpillars. The rate at which caterpillars were delivered to the nestlings (in mg:nestling:h) was strongly correlated with the caterpillar biomass available (in mg:m of branches) and nestling growth rate was significantly influenced by the mass of available caterpillars. The results provide evidence why perfect timing of breeding is so important for the Great Tit, and contribute to the understanding of the causal link between food supply, growth and breeding success.
Content may be subject to copyright.
JOURNAL OF AVIAN BIOLOGY 31: 206214. Copenhagen 2000
Prey selection and foraging performance of breeding Great Tits
Parus major in relation to food availability
Luzia Naef-Daenzer, Beat Naef-Daenzer and Ruedi G. Nager
Naef-Daenzer, L., Naef-Daenzer, B. and Nager, R. G. 2000. Prey selection and
foraging performance of breeding Great Tits Parus major in relation to food
availability. – J. Avian Biol. 31: 206 214.
We studied the nestling diet and the foraging performance of Great Tits in relation
to prey abundance in the field. Numerous experimental studies present data on
foraging decisions in captive Great Tits. Little is, however, known about prey
selection in the field in relation to the food available and the consequences this has
for the food delivery rate to nestlings. Since the foraging performance of the parents
is one of the main determinants of fledging weight and juvenile survival, foraging
behaviour is an important part of Great Tit reproduction. During the early breeding
season up to 75% of the prey biomass delivered to the nestlings were spiders, which
is in contrast with other studies. Only when caterpillars reached a size of 1012 mg
(approximately the average size of the spiders caught at that time) did the Great Tits
change their preferences and 8090% of the delivered prey masses were caterpillars,
as reported by other authors. This ‘switching’ between prey occurred within a few
days. It was not related to the changes in abundance but to size of caterpillars. The
rate at which caterpillars were delivered to the nestlings (in mg/nestling/h) was
strongly correlated with the caterpillar biomass available (in mg/m of branches) and
nestling growth rate was significantly influenced by the mass of available caterpillars.
The results provide evidence why perfect timing of breeding is so important for the
Great Tit, and contribute to the understanding of the causal link between food
supply, growth and breeding success.
L.Naef-Daenzer,B.Naef -Daenzer,Swiss Ornithological Institute,CH -
:,Ornithology Group,Gra-
ham Kerr Building,Institute of Biomedical and Life Sciences,Uni6ersity of Glasgow,
Glasgow G
12 8
The timing of the breeding season of birds is ultimately
adjusted to the availability of their food (Lack 1968).
Among the best studied systems are tits (Parus spp.) of
temperate deciduous forests. Their main prey during
breeding is various species of caterpillars living in tree
foliage. It is now commonly observed that birds raising
their young at times with highest caterpillar abundance
have the highest reproductive output (e.g. Tinbergen
and Boerlijst 1990, Nager and van Noordwijk 1995, van
Noordwijk et al. 1995, Dias and Blondel 1996). To
understand the seasonal pattern in reproductive success
we need to understand the proximate mechanisms that
link foraging decisions and fitness. Higher food
availability results in better growth (Keller and van
Noordwijk 1994), and better growth improves survival
and recruitment chances of the offspring (Tinbergen
and Boerlijst 1990, Gebhardt and van Noordwijk 1991,
Verboven and Visser 1998). However, the underlying
mechanism of this relationship between food availabil-
ity and breeding success for Great Tits Parus major
remains largely unexplored.
How prey selection and searching behaviour are af-
fected by the availability and quality of prey has been
discussed extensively at the theoretical level (Charnov
1976, Lessells and Stephens 1983, Stephens and Krebs
1986). Although there are excellent studies on foraging
decisions in captive Great Tits (e.g. Smith and Sweat-
man 1974, Krebs et al. 1977, 1978, Ydenberg 1984)
little is known about foraging decisions in relation to
food availability in natural conditions. To discuss prey
selection in relation to abundance an assessment of
both variables is necessary. Earlier studies have de-
scribed the nestling diet of Great Tits, but the seasonal
development of prey species available has not been
investigated with high temporal resolution (Tinbergen
1960, Gibb and Betts 1963, Royama 1970, van Balen
1973). Several more recent field studies on various bird
species analyse the foraging decisions in relation to prey
abundance in general (Great Tits, e.g. Barba and Gil-
Delgado 1990, Przbylo 1995, Riddington and Gosler
1995, Massa and Lo Valvo 1996; other bird species, e.g.
Krebs and Avery 1985, Brodmann 1994, Frey-Roos et
al. 1995, Illera and Atienza 1995). Detailed field studies
on prey selection including an assessment of prey
availability concerned other passerines and non-passer-
ines (Davies 1977, Goss-Custard 1977b, Turner 1982,
Thompson 1984). Here we present a study on the
nestling diet of Great Tits in relation to the seasonal
variation in prey availability, and its effect on the rate
at which energy is delivered to the brood. The investi-
gation considers two levels of analysis: (1) prey selec-
tion in relation to size and density of prey species and
(2) the foraging performance of tit parents in relation to
prey biomass available.
Data were collected from 27 April to 19 May 1990,
which is the main breeding season in the area, in a
mixed deciduous forest of 80 ha near Basel, Switzer-
land, 270 m a.s.l. Thirty percent of the forest canopy
trees are oaks (Quercus petraea,Q.robur). The remain-
der of the canopy includes ash Fraxinus excelsior (36%),
beech Fagus sil6atica (18%), and hornbeam Carpinus
betulus (10%). Since 1986 about 300 nestboxes had been
present in the forest.
Ten boxes were equipped with a photo-recorder con-
sisting of a Super 8 mm movie camera, a photocell
recording each passage of a bird, and an infrared
flashlight. Approximately 90% of all passages were
recorded. Failures occurred when the departure instead
of the entrance of a bird was recorded or when the flash
was insufficiently charged. A watch mounted inside the
nestbox gave the exact time when the pictures were
taken. The cameras were kept for 1 to 3 days at a given
box. To allow the birds to recover from the disturbance
and resume their normal feeding rate, records from the
first hour after camera installation and after daily in-
spection of the nestboxes were omitted. To avoid ad-
verse effects or premature fledging, observations were
restricted to broods where nestlings were 712 days
old. The brood sizes were 3, 4, 5, 6, 8 (one brood each)
and 7 (5 broods), respectively; the mean brood size was
To analyse the effects of foraging conditions on
nestling growth, we used the method of growth analysis
developed by Keller and van Noordwijk (1993, 1994).
This method quantifies growth rates as a ratio of the
observed daily growth compared to the expected
growth under good conditions. Details of the technique
are given in Keller and van Noordwijk (1993). Here, we
give only a brief summary. Expected growth is evalu-
ated on the basis of a Richards growth function in
which the winter weights of parents were used as an
estimate of the asymptote of individual growth curves.
The ratio of the realised over the expected growth R is
independent of age and actual size and is used as an
estimator of the foraging conditions. R equals 1 if a
nestling grows as expected under good conditions.
We examined the photographs under a binocular
microscope (1025 ×), determined the feeding time
and identified the prey species. We categorised the prey
as caterpillars (mainly Tortricidae, but also Geometri-
dae and Noctuidae), spiders, beetles (order Coleoptera),
and ‘other winged insects’, and ‘unidentified prey’. A
large proportion of unidentified prey consisted mainly
of a whitish mass similar to butter, probably some
fungus. The body length of caterpillars and spiders was
estimated by comparing it to the visible part of the
Great Tits bill, which had an average estimated length
of approximately 8 mm. Five size categories (multiples
of bill length) were discerned (58mm,9mm16mm,
17 mm24 mm, 25 mm 32 mm, ]32 mm). The mass
of caterpillars was estimated from an empirical calibra-
tion curve derived from caterpillars collected from 401
branch samples, as described below. The mass of spi-
ders was calculated using the equation y=0.35x
where y is the mass in mg and x is body length in mm
(Lille 1996). As caterpillars and spiders this time of the
year contain a similar amount of water (around 74%
and 69%, respectively, Brodmann 1994), we refer to
fresh weights. As ‘winged insects’ were not identified to
the level of species, this category comprises animals of
quite different shapes, and no estimate of size was
We used a skylift reaching 35 m in height to assess
prey composition and density in the canopy of trees,
which is the main foraging habitat during breeding
(Glutz von Blotzheim and Bauer 1993). Two to six
samples were taken weekly from 30 trees for six weeks.
The branches were removed after enclosing them in a
800 l plastic bag. Samples were stored at field tempera-
tures and examined within four days of collection.
Samples that were examined later than four days after
collection were excluded from the analysis. Caterpillars
and other invertebrates were counted and stored in
alcohol. The lengths of the twigs with a diameter of less
than 1 cm (i.e. the parts carrying leaves), were measured.
For 401 branch samples, the mass and length of all
caterpillars were determined to the nearest mg and mm,
respectively. From these data a calibration curve relat-
ing these two measures was determined (Fig. 1). Dates
are given here as continuous numbers with 1 March=
1. The mean caterpillar hatching date in 1990 was 21
Fig. 1. Relationship between length (mm) and mass (mg, fresh
weight) of caterpillars on the four tree species (y=0.08x
r=0.95, n=1412). The data stem from 401 branch samples
collected at the study site during the study.
Fig. 2. Density (number of prey per m branch) of four prey
types on trees and their proportion in the nestlings’ diet during
the course of the breeding season. Each triangle is the average
for one nest. The averages per day and nest (dots) are also
shown. The density scale is given on the right y-axis, the
proportion scale on the left y-axis. Note the scale differences
between the prey species on the y-axes.
March. All statistical analyses are based on averages
per box. The figures also show the daily averages.
Nestling diet
The Great Tit is usually a single-prey loader; 6.4%9
10.7 (median9s.e.) of all prey were brought in double-
prey loads. Caterpillars and spiders were the major
components of the nestling diet, winged insects con-
tributed up to 20% and beetles rarely more than 2% by
prey number (Fig. 2). Unidentified prey were more
important before day 68, when they accounted for up
to 20% of prey items; on average 8.9%98.9% (s.e.) of
all prey remained unidentified. From the end of April
to mid May considerable changes in the composition of
the nestling diet were observed. Birds nesting early fed
their young mainly spiders (Fig. 2); by day 60 (end of
April), 6070% by number of diet items fed to the
nestlings were spiders but by day 63 spiders only ac-
counted for 10% of the nestling diet. Simultaneously
the proportion of caterpillars in the nestling diet in-
creased from 20% to 6080%, and for the rest of the
observation period caterpillars dominated the nestling
food. Winged insects were brought in variable propor-
tions. Beetles constituted up to 4% of the early nestling
diet but after day 63 dropped to almost zero.
If we consider prey numbers, the pattern observed in
the nestling diet mostly did not reflect the abundance of
the prey types in the environment. Whereas the propor-
tion of caterpillars in the nestling diet increased, the
caterpillar density in the trees declined over the same
period (Fig. 2). Similarly, whereas the density of spiders
in the trees showed a threefold increase over the obser-
vation period, their proportion in the nestling diet
decreased dramatically.
Winged insects were taken in proportions which did
not reflect their density on trees either. The very small
proportion of beetles in the diet was not related to
their density on trees. Only before day 65 were they
collected to any extent.
As a measure of the preference of Great Tits for a
particular prey type we used its relative frequency in
the diet (number of prey x taken/total number of prey
taken) compared to its relative frequency on trees
(number of prey x available on trees/total number of
potential prey available on trees). Early in the season
caterpillars were caught less frequently than expected
from their abundance on trees (Fig. 3); the tits’ prefer-
ence increased continuously, until after day 63 they
were caught approximately according to their relative
frequency on trees, and after day 73 they were strongly
preferred. In contrast, spiders were strongly preferred
during the early season; this preference decreased con-
tinuously until day 73 from which date on they were
caught according to their relative frequency on the
trees. Winged insects were always taken much below
their relative frequency on trees.
Prey size selection
Early in the season Great Tits selected caterpillars well
above the mean caterpillar size available; the mass of
the average caterpillar delivered to a brood was 2.6
times the mass of caterpillars on trees (Fig. 4). For
example, on day 65 the mean weight of the caterpillars
on trees was 18 mg, whereas the tits fed their young
with caterpillars of an average weight of 46 mg. The
mean size of caterpillars available peaked at day 73
and then decreased due to the hatching of other cater-
pillar species. From day 67 to 80 the selected caterpil-
lars were still heavier than the average caterpillar on
trees, but the selection for the biggest items was less
marked than earlier in the season.
Great Tits switched from spiders as their main prey
type to caterpillars when, around day 62, the mean
weight of caterpillars on trees reached c. 15 mg (Fig.
5) and the caterpillars selected for the nestlings
weighed around 40 mg (Fig. 6); the caterpillars now
amounted to 6080% (by number) of the diet. Al-
though spiders contributed less than 10% of the diet
for the rest of the observation period, they were, in
relation to their density, still very much preferred (Fig.
3). Overall the mean weight of caterpillars was greater
than the mean weight of spiders (Fig. 6; ANCOVA,
corrected for date, df=1; 17, F=10.15, p B0.005).
The preference for a prey type and the switching were
not related to the age of the nestlings within the age
span studied (712 days), but to the date, which repre-
sents the development of the prey species on the trees
(Fig. 7). Early in the season the nestling diet contained
a high percentage of spiders for all nestling ages, and
later the percentage was low even for nestlings aged 7
Foraging performance
The amount (mg) of caterpillars the Great Tit parents
delivered per hour and per nestling was correlated with
Fig. 3. Proportions of three different prey species in the diet of
the nestlings in relation to their proportions on trees. Triangles
represent averages per nest, dots averages per day and nest.
Note the linear scale on the y-axis for ‘winged insects’. A value
above 1 means that the prey species was taken more often than
expected from its relative frequency on trees.
Fig. 4. Mean weight (mg) of caterpillars on trees (solid line),
75% percentile (dashed line) and maximum weight of caterpil-
lars on trees (dotted line), respectively, and weight of caterpil-
lars brought to the nest. Triangles represent averages per nest,
dots averages per day and nest. The average mass of caterpil-
lars on trees increased until about day 74. Then the earliest
species pupated and small new caterpillars appeared, so that
the average caterpillar mass diminished.
Fig. 6. Seasonal development of the mass of caterpillars and
spiders brought to the nest. The values for caterpillars are
represented by triangles (y=10.99+0.45x, r=0.46); the val-
ues for spiders are represented by dots (y=11.61=0.31x,
r=0.34). Big symbols represent averages per box, small sym-
bols averages per day and box. The masses of caterpillars and
spiders differ significantly (ANCOVA, df=1; 17, F=10.15,
the development of the caterpillar biomass (mg/m) on
the trees (Fig. 8; y=−208.76 +195.93 · log x, r =0.88,
The mass of caterpillars delivered per hour (mg cater-
pillars/hour) was influenced by caterpillar biomass and
brood size (Table 1). These two variables explain 83%
of the variance. Thus the rate at which Great Tits
delivered prey to the brood was influenced by both the
abundance of their main food at the time and the
number of young they had to feed.
We found a strong relationship between the average
nestling growth rate R per brood and the size (mg) of
available caterpillars (Fig. 9; y
=1.288.05/x, n=
10, r=0.92, pB0.0001). This result indicates that the
availability of large prey has an immediate effect on the
growth performance of tit nestlings. Furthermore, it
confirms radiotracking data (Naef-Daenzer and Keller
1999) showing that the average search time per feeding
is a function of both caterpillar size and density.
Feeding frequency
As the mass of prey items varied considerably, feeding
frequency does not provide accurate information about
the amount of food delivered to the young. The feeding
frequency per hour was negatively influenced by date
and positively by brood size (Table 2). The two vari-
ables explain 81% of the variance. Tits with larger
broods and tits breeding early, when caterpillars were
small and nestlings were mainly fed spiders, had to
bring food more often. The feeding frequencies are
Fig. 5. Relationship between the proportions of caterpillars
and spiders in the nestling diet in relation to the average
weight of caterpillars on trees. Values for caterpillars are
represented by triangles, values for spiders by dots. Big sym-
bols represent averages per box, small symbols averages per
day and box.
Fig. 7. Percent of spiders (in
numbers) in the diet in relation to
date and nestling age. The averages
per day and nest are shown. The
amount of spiders in the diet is not
dependent on nestling age.
Fig. 8. The mass of caterpillars the parents brought per
nestling and hour as a function of caterpillar biomass on trees.
Big symbols represent averages per box, small symbols aver-
ages per day and box. y=−208.76 +195.93 · log x, r=0.88,
comparable to those observed at the same time for
radio-tagged birds (Naef-Daenzer and Keller 1999).
By increasing the feeding frequency parents with
larger broods were able to maintain an almost constant
level of food provisioning per nestling (Fig. 10). The
above result showing that the brood size positively
influences the amount (mg) of caterpillars delivered per
hour points in the same direction. Fig. 10 shows also
how, in an early brood, caterpillars were substituted by
an almost equal amount of spiders.
Numerous field studies describe the nestling diet of
Great Tits in various habitats (e.g. Betts 1955, Gibb
and Betts 1963, van Balen 1973, Cowie and Hinsley
1988, Barba and Gil-Delgado 1990). In contrast to
them we additionally investigated prey abundance in
detail. This enabled us to relate foraging decisions and
performance to food availability. The application of
foraging theory to wild animals has been criticised
because it introduces simplifications and avoids con-
straints in models of animals’ foraging behaviour (Lu-
cas 1987, Grundel 1990). The assumptions of foraging
theory are often not met in the field situation. For
example, only some prey species are evenly distributed,
while others show a clumped distribution, as our
branch samples revealed. But the main drawback of the
assumptions is that, unlike in the laboratory, prey
species in the wild grow and they move between
patches. Thus prey encounter rates keep changing in
time and space. Therefore, in order to forage optimally
an animal needs to collect information about food
availability. Many models, however, assume a forager
either to have such information or to forage at random.
Observations of radio-tagged Great Tits provide evi-
dence that the birds return several times in sequence to
profitable patches, but keep exploring other sites con-
tinuously (Naef-Daenzer and Keller 1999).
Although many possible factors were not controlled
for in this field study, some rules proposed by foraging
theory were confirmed: the tits preferred larger prey to
smaller and much more frequent prey. At the beginning
of observations spiders were bigger and preferred to the
caterpillars, which were much more numerous but
small. Although spiders contributed little to the diet
later in the season, in relation to their relative density
they were preferably collected throughout the study.
Royama (1970) proposed for Parus major minor in
Japan that young nestlings might need spiders because
these contain some nutrient important for their devel-
opment. He observed the highest spider proportion in
the diet of nestlings of ages 56 days. However,
Royama presented no data on the seasonal abundance
of prey species. Our results indicate for the ages 712
days that the changes in the diet were entirely related to
the seasonal development of prey biomass. Grundel and
Dahlsten (1991) confirmed Royama’s observations for
Mountain Chickadees Parus gambeli. In contrast to
this, Betts’ observations (1955) of the diet of one brood
of Great Tits look similar to our data for the age 7 days
onwards. Other authors have shown for other bird
species that food items are not only selected to max-
imise the amount of food brought to nestlings but also
its quality (Goss-Custard 1977a, Krebs and Avery
1985). In contrast, Brodmann (1994) found in his de-
tailed study of Water Pipits Anthus spinoletta that food
quantity was more important than quality: the pre-
ferred prey species did not differ in regard to lipid,
carbohydrate, water or energy content compared to
other possible prey species present. The number of
fledglings was correlated with the amount of food
available, but not with the quality of prey. We show
Fig. 9. Relationship between caterpillar mass (mg) and mean
nestling growth rate R per nest. y
=1.288.05/x, r=0.92,
that by using the more profitable spiders early in the
season the tits could in spite of the low caterpillar
weights – maintain an almost equally high rate of food
delivery (mg prey per hour and nestling) as could
parents taking caterpillars later in the season.
Throughout the observations the tits selected cater-
pillars which were by far heavier than the average
caterpillars on trees. Many authors have described this
selection for large size of prey in the nestling diet (e.g.
Tinbergen 1960, Gibb and Betts 1963, Davies 1977,
Brodmann 1994). A field study with radio-tagged tits
indicated that big caterpillars were found faster than
smaller ones (Naef-Daenzer and Keller 1999). The re-
duced searching time and the higher energy gain per
feeding trip make large caterpillars more profitable.
The tits’ diet was not influenced by the abundance of
winged insects, which had comparable densities to
caterpillars and were an order of magnitude more fre-
quent than spiders and beetles. Nevertheless winged
insects were caught in remarkably lower proportions
than caterpillars, and their proportion in the diet was
much below the frequency expected from their relative
density on trees. One reason might be that this group
Table 1. Multiple regression of caterpillar mass (mg) deliv-
ered per hour (dependent variable) in relation number of
nestlings and caterpillar biomass on trees. R
n=10 broods.
Coefficient SE p
Constant 705.91 219.47 0.015
Brood size 168.60 31.82 0.001
1.818.22 0.003Caterpillar biomass
Table 2. Multiple regression of nestling feeding frequency per
hour (dependent variable) and date, brood size and caterpillar
size on trees. R
(adj.)=0.81, n=10 broods.
Coefficient SE p
Constant 77.83 17.31 0.004
Brood size 4.84 1.22 0.007
0.002Date 1.44 0.27
0.17Caterpillar size 1.38 0.89
Fig. 10. Relationship between brood size and mg caterpillars
per hour and nestling, and mg spiders per hour and nestling
delivered by the parents. The values for caterpillars are repre-
sented by triangles, values for spiders by dots. Big symbols
represent averages per box, small symbols averages per day
and box. The very low caterpillar delivery rates for the nest
with 5 young were observed at the beginning of the observa-
tion period, when spiders were the main part of the diet.
post-fledging survival (Tinbergen and Boerlijst 1990,
Gebhardt and van Noordwijk 1991, Verboven and
Visser 1998, Naef-Daenzer and Keller 1999). Although
there were indications that tits compensate for in-
creased demands of the brood, the energy flow to the
nest was mainly determined by the availability of prey
and much less so by the behaviour of the parents. Early
in the season the Great Tits substituted the small
caterpillars by an almost equal amount of spiders, but
the growth rate of their young was lower than during
the caterpillar peak. This makes clear that the parents’
performance is limited by the availability of profitable
prey, in this case, big caterpillars. The timing of the
brood with the time of maximum size and abundance
of caterpillars is therefore of greatest importance.
Acknowledgements We would like to thank S. Gebhardt-
Henrich for her assistance in the field work. A. van Noordwijk
kindly offered the data for analysis. L. Keller provided growth
data for the observed broods. V. Keller and N. Zbinden gave
helpful comments on the manuscript. Financial support from
the Swiss National Science Foundation (grant number 3.153-
1.88) is gratefully acknowledged.
van Balen, J. H. 1973. A comparative study of the breeding
ecology of the Great Tit Parus major in different habitats.
Ardea 61: 1– 93.
Barba, E. and Gil-Delgado, J. A. 1990. Seasonal variation in
nestling diet of the Great Tit Parus major in orange groves
in eastern Spain. – Ornis Scand. 21: 296 298.
Barba, E., Albano Lopez, J. and Gil-Delgado, J. A. 1996. Prey
preparation by adult Great Tits Parus major feeding
nestlings. – Ibis 138: 532 538.
Betts, M. M. 1955. The food of titmice in oak woodland. J.
Anim. Ecol. 24: 282323.
Brodmann, P. A. 1994. Relations between food conditions,
foraging behaviour and reproductive performance in the
water pipit (Anthus sp. spinoletta). Ph.D. Thesis, Univer-
sity of Zu¨rich, Switzerland.
Charnov, E. L. 1976. Optimal foraging: The marginal value
theorem. Theor. Popul. Biol. 9: 129 136.
Cowie, R. J. and Hinsley, S. A. 1988. Feeding ecology of
Great Tits (Parus major) and Blue Tits (Parus caeruleus ),
breeding in suburban gardens. J. Anim. Ecol. 57: 611
Davies, N. B. 1977. Prey selection and the search strategy of
the spotted flycatcher (Muscicapa striata): a field study on
optimal foraging. Anim. Behav. 25: 1016 1033.
Dias, P. C. and Blondel, J. 1996. Breeding time, food supply
and fitness components of Blue Tits (Parus caeruleus)in
Mediterranean habitats. Ibis 138: 644 649.
Frey-Roos, F., Brodmann, P. A. and Reyer, H. U. 1995.
Relationship between food resources, foraging patterns,
and reproductive success in the water pipit, Anthus sp.
spinoletta. Behav. Ecol. 6: 287 295.
Gebhardt-Henrich, S. G. and van Noordwijk, A. J. 1991.
Nestling growth in the Great Tit I. Heritability estimates
under different environmental conditions. J. Evol. Biol.
3: 341362.
Gibb, J. A. and Betts, M. M. 1963. Food and food supply of
nestling tits (Paridae) in Breckland pine. J. Anim. Ecol.
32: 489533.
Glutz von Blotzheim, U. and Bauer, K. M. 1993. Handbuch
der Vo¨gel Mitteleuropas Vol. 13. Aula, Wiesbaden.
comprises different species; their highly variable pro-
portion in the diet makes it probable that they were
used as a supplement in the diet. That winged insects,
such as Noctuidae, can even be the main food of
nestlings when caterpillars are scarce was shown for
orange groves in Spain by Barba and Gil-Delgado
(1990). The wings of Noctuidae might make them
difficult to catch and require a longer handling time.
Barba et al. (1996) observed that the tits removed at
least half of the legs and wings of 71% of Lepidoptera
imagines fed to nestlings.
The Great Tits switched from one most profitable
prey type, spiders, to a new most profitable prey type,
caterpillars. This switching occurred at the moment
when the caterpillars got heavier than the spiders. Fi-
nally, early in the season when big caterpillars, the most
profitable prey, were rare, the Great Tits’ diet was
The present analysis quantifies the influence of the
available prey biomass on the mass of food delivered
per hour to the nestlings and provides evidence that tit
parents depend to a great extent on the foraging condi-
tions in the habitat. It confirms by direct measurement
the mechanism by which food availability influences the
growth rate of the nestlings, which in turn is related to
Goss-Custard, J. D. 1977a. The energetics of prey selection by
redshank, Tringa totanus (L.) in relation to prey density.
J. Anim. Ecol. 46: 119.
Goss-Custard, J. D. 1977b. Optimal foraging and the size
selection of worms by redshank, Tringa totanus,inthe
field. Anim. Behav. 25: 10 29.
Grundel, R. 1990. The role of dietary diversity, prey capture
sequence and individuality in prey selection by parent
mountain chickadees (Parus gambeli ). – J. Anim. Ecol. 59:
Grundel, R. and Dahlsten, D. L. 1991. The feeding ecology of
mountain chickadees (Parus gambeli ): patterns of
arthropod prey delivery to nestling birds. Can. J. Zool.
69: 17931804.
Illera, J. C. and Atienza, J. C. 1995. Foraging shifts by the
Blue Tit (Parus caeruleus) in relation to arthropod
availability in a mixed woodland during the springsum-
mer period. Ardeola 42: 39 48.
Keller, L. and van Noordwijk, A. J. 1993. A method to isolate
environmental effects on nestling growth, illustrated with
examples from the Great Tit (Parus major L.). – Funct.
Ecol. 7: 493502.
Keller, L. and van Noordwijk, A. J. 1994. Effects of local
environmental conditions on nestling growth in the Great
Tit (Parus major). – Ardea 82: 349 362.
Krebs, J. R., Erichsen, J. T., Webber, M. I. and Charnov, E.
L. 1977. Optimal prey selection in the Great Tit (Parus
major). Anim. Behav. 25: 3038.
Krebs, J. R., Kacelnik, A. and Taylor, P. 1978. Test of
optimal sampling by foraging Great Tits. Nature 275:
Krebs, J. R. and Avery, M. I. 1985. Central place foraging in
the European Bee-eater, Merops apiaster. J. Anim. Ecol.
54: 459472.
Lack, D. 1968. Ecological Adaptations for Breeding in Birds.
Methuen, London.
Lessells, C. M. and Stephens, D. W. 1983. Central place
foraging: Single-prey loaders again. Anim. Behav. 31:
Lille, R. 1996. Zur Bedeutung von Brachefla¨chen fu¨r die
Avifauna der Agrarlandschaft: Eine nahrungso¨kologische
Studie an der Goldammer Emberizia citrinella. – Paul
Haupt, Bern.
Lucas, J. R. 1987. The influence of time constraints on diet
choice of the Great Tit Parus major. Anim. Behav. 35:
Massa, B. and Lo Valvo, F. 1996. Arthropod abundance and
breeding performance of tits in deciduous, evergreen oak-
woods and pine reafforestation of Sicily (Italy). Avocetta
20: 113124.
Naef-Daenzer, B. and Keller, L. 1999. Foraging performance
of Great and Blue Tits (Parus major,Parus caeruleus)in
relation to caterpillar development and its consequences
for nestling growth and fledging weight. J. Anim. Ecol.
68: 708718.
Nager, R. G. and van Noordwijk, A. J. 1995. Proximate and
ultimate aspects of phenotypic plasticity in timing of Great
Tit breeding in a heterogeneous environment. Am. Nat.
146: 454474.
van Noordwijk, A. J., McCleery, R. H. and Perrins, C. M.
1995. Selection for the timing of Great Tit breeding in
relation to caterpillar growth and temperature. J. Anim.
Ecol. 64: 451458.
Przbylo, R. 1995. Intersexual niche differentiation – field
data on the Great Tit Parus major. J. Avian Biol. 26:
Riddington, R. and Gosler, A. G. 1995. Differences in repro-
ductive success and parental qualities between habitats in
the Great Tit Parus major. Ibis 137: 371 378.
Royama, T. 1970. Factors governing the hunting behaviour
and selection of food by the Great Tit (Parus major).–J.
Anim. Ecol. 39: 619668.
Smith, J. N. M. and Sweatman, H. P. A. 1974. Food-searching
behaviour of titmice in patchy environments. Ecology
55: 12161232.
Stephens, D. W. and Krebs, J. R. 1986. Foraging Theory.
Monographs in Behaviour and Ecology, Princeton Univer-
sity Press, Princeton, New Jersey.
Tinbergen, L. 1960. The natural control of insects in
pinewoods. I. Factors influencing the intensity of predation
by songbirds. Arch Neerl. Zool. 13: 265 343.
Tinbergen, J. M. and Boerlijst, M. C. 1990. Nestling weight
and survival in individual Great Tits (Parus major).–J.
Anim. Ecol. 59: 11131127.
Thompson, D. B. A. and Barnard, C. J. 1984. Prey selection
by plovers: optimal foraging in mixed-species groups.
Anim. Behav. 32: 554563.
Turner, A. K. 1982. Optimal foraging by the swallow (Hirundo
rustica, L.): prey size selection. Anim. Behav. 30: 862
Verboven, N. and Visser, M. 1998. Seasonal variation in local
recruitment of great tits: the importance of being early.
Oikos 81: 511524.
Ydenberg, R. C. 1984. Great Tits and giving-up times: deci-
sion rules for leaving patches. Behaviour 90: 1 24.
... Against our hypothesis, we found no significant association between nestling body mass and brood size: neither reduction nor enlargement of the broods resulted in significant body mass differences in the nestlings on day 7 and day 14 post-hatch. While the result is supported by some studies in which associations between nestling body mass and brood size have been tested [61,98], the majority of the literature shows that brood size negatively correlates with nestling body mass: in larger broods nestlings are generally of lower mass [52,53,57,67,[99][100][101][102][103][104]]. ...
... In that case the number of nestlings transferred between enlarged and reduced nests should probably have been larger to create differences in nestling body mass between the two treatments. Still, we think that the decision to transfer + 2/− 2 was reasonable since it was based on extensive evidence from previous studies [103]. Secondly, it could be that the enlarged brood size negatively influences some other physiological traits while body mass was retained at the expense of these other traits e.g., immune system functioning [105,106]. ...
... This could be a result of changes in the food items that great tits use, changes in temperature conditions or in parental investment during the breeding season. As the season progresses, the abundance of insect taxa varies, and this can result in changes in nutrient rich food [103,107]. For example, great tits can select certain lepidopteran larvae that vary in their abundance during the great tit breeding season [108]. ...
Full-text available
Background The gut microbiome forms at an early stage, yet data on the environmental factors influencing the development of wild avian microbiomes is limited. As the gut microbiome is a vital part of organismal health, it is important to understand how it may connect to host performance. The early studies with wild gut microbiome have shown that the rearing environment may be of importance in gut microbiome formation, yet the results vary across taxa, and the effects of specific environmental factors have not been characterized. Here, wild great tit (Parus major) broods were manipulated to either reduce or enlarge the original brood soon after hatching. We investigated if brood size was associated with nestling bacterial gut microbiome, and whether gut microbiome diversity predicted survival. Fecal samples were collected at mid-nestling stage and sequenced with the 16S rRNA gene amplicon sequencing, and nestling growth and survival were measured. Results Gut microbiome diversity showed high variation between individuals, but this variation was not significantly explained by brood size or body mass. Additionally, we did not find a significant effect of brood size on body mass or gut microbiome composition. We also demonstrated that early handling had no impact on nestling performance or gut microbiome. Furthermore, we found no significant association between gut microbiome diversity and short-term (survival to fledging) or mid-term (apparent juvenile) survival. Conclusions We found no clear association between early-life environment, offspring condition and gut microbiome. This suggests that brood size is not a significantly contributing factor to great tit nestling condition, and that other environmental and genetic factors may be more strongly linked to offspring condition and gut microbiome. Future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental influences.
... In England, the Yellow Wagtail prefers small prey with the mean length 7 mm (Davies 1977). Overall analyses of prey size in this species have not been reported, but in the most cases the mean prey size increases with the age of the young in passerines (Naef-Daenzer et al. 2000) and for energetic reasons the mean prey tends to be longer than the mean bill length (Brandl et al. 1994). ...
... The Yellow Wagtail therefore belongs to the group of so-called "multiple prey loaders" (Orians & Pearson 1979, Houston 1985, Gaglio et al. 2018). The number of prey items per feeding thus significantly differs between the species bringing mainly one prey item per feeding ("single prey loader"), such as e. g. the Great Tit Parus major (Naef-Daenzer et al. 2000, Barba et al. 2009 al. 2016), raptors (Chavko & Krištín 2017) and other bird taxa, which feed with a higher number of prey items per feeding only exceptional. We found that higher number of prey items (5-10 in the bill) per feeding consisted mainly of the same prey taxa, e. g. smaller (<10 mm) cicadas (Cicadellidae), but also larger (≥25 mm) Geometridae caterpillars (Fig. 4), goldeneyes (Chrysopa sp.) and tipulids (Tipulidae). ...
... Only a small fraction of the tested birds actually attacked the dummies. This could suggest that I. podalirius butterflies are not the usual prey consumed by great tits [41], especially during the season when the tests were carried out (late autumn and winter), where they mostly rely on seeds rather than on insects. Our behavioural experiments are thus relevant for the behaviour of generalist predators that are probably naive to the phenotypes of the tested butterflies, a likely situation in nature, as no specialist predator is known for I. podalirius. ...
Predation is a powerful selective force shaping many behavioural and morphological traits in prey species. The deflection of predator attacks from vital parts of the prey usually involves the coordinated evolution of prey body shape and colour. Here, we test the deflection effect of hindwing tails in the swallowtail butterfly Iphiclides podalirius . In this species, hindwings display long tails associated with a conspicuous colour pattern. By surveying the wings within a wild population of I. podalirius , we observed that wing damage was much more frequent on the tails. We then used a standardised behavioural assay employing dummy butterflies with real I. podalirius wings to study the location of attacks by great tits Parus major . Wing tails and conspicuous coloration of the hindwings were struck more often than the rest of the body by birds. Finally, we characterised the mechanical properties of fresh wings and found that the tail vein was more fragile than the others, suggesting facilitated escape ability of butterflies attacked at this location. Our results clearly support the deflective effect of hindwing tails and suggest that predation is an important selective driver of the evolution of wing tails and colour pattern in butterflies.
... While some studies have looked at the effects of urbanization gradients and foraging behavior on avian reproduction (e.g., Caizergues et al. 2021;Jarrett et al. 2020), a high-resolution analysis of the biotic components in the immediate surroundings of breeding urban birds, here referred to as territory, and its effects on avian reproduction across years is still lacking (but see Narango et al. 2018). Analyzing the effects of tree composition on breeding success across multiple years could render important answers, as the availability of caterpillars and other invertebrates are often closely linked to the availability of host plants, in addition to being annually variable (van Asch and Visser 2007;Mutshinda et al. 2011). Furthermore, vegetation itself is one of the most important aspects for maintaining biodiversity in urban environments (Beninde et al. 2015). ...
Full-text available
Birds breeding in urban environments have lower reproductive output compared to rural conspecifics, most likely because of food limitation. However, which characteristics of urban environments may cause this deficiency is not clear. Here, we investigated how tree composition within urban territories of passerine birds is associated with breeding probability and reproductive success. We used 7 years of data of breeding occupancy for blue and great tits ( Cyanistes caeruleus; Parus major ) and several reproductive traits for great tits, from 400 urban nest boxes located in 5 parks within the city of Malmö, Sweden. We found that tits, overall, were less likely to breed in territories dominated by either non-native trees or beech trees. Great tit chicks reared in territories dominated by non-native trees weighed significantly less, compared to territories with fewer non-native trees. An earlier onset of breeding correlated with increased chick weight in great tits. Increasing number of common oak trees ( Quercus robur ) was associated with delayed onset of breeding in great tits. Notably, as offspring survival probability generally increased by breeding earlier, in particular in oak-dominated territories, our results suggest that delayed onset of breeding induced by oak trees may be maladaptive and indicate a mismatch to this food source. Our results demonstrate that tree composition may have important consequences on breeding success of urban birds, but some of these effects are not consistent between years, highlighting the need to account for temporal effects to understand determinants of breeding success and inform optimal management in urban green spaces.
... Little is known about the foraging strategy and diet differences between floaters and territorial birds of large raptor species and how they choose prey and perform feeding behavior (however, see [15]). It is well known that the availability and accessibility of food resources are key factors that shape the foraging behavior and dietary choice [2,6]. Top predators can adapt their diet mainly in response to habitat alteration and the depletion of main food resources [7,55,56], but also to avoid competition [8]. ...
Full-text available
The Optimal Foraging Theory predicts that, to maximize fitness, animals adapt their foraging strategy that provides the most benefit for the lowest cost, maximizing the net energy gained. While the diet of many breeding raptor populations is well known, studies on the foraging patterns of non-territorial birds of prey (floaters) are scarce. In this study, we examined the foraging pattern of non-territorial Eastern Imperial Eagle, scrutinizing different aspects of its feeding ecology and behavior. We built a simple model of the optimal foraging strategy of floater eagles including the success of foraging as a currency as well as environmental factors such as seasons, type of prey, habitat, foraging techniques, and eagle age as a limitation affecting the foraging efficiency of birds. We found that floaters focused their diet exclusively on European Souslik, accounting for almost half (44.2%) of the eagle’s prey. Diet differences between floaters and breeders were due to higher Souslik and carrion consumption and lower Hedgehog predation by floater eagles. The diet diversity of breeding eagles (H = 3.297) was much higher than that of floaters (H = 1.748). Our model suggested that the foraging mode, habitat type, and season best explained the feeding success of non-territorial eagles (ΔAIC = 0.00, w = 0.42). Of all explanatory factors, “Kleptoparasitism” (β2 = −4.35), “Rodents” (β2 = −4.52), “Pasture” (β2 = 2.96), “Wheat” (β2 = 4.41), “In the air” (β2 = 4.16), and “Other habitats” (β2 = 4.17) had a pronounced effect. The factors “Spring–summer season” (β2 = −0.67) and “European Souslik” (β2 = −2.76) had a marginal effect in our models. Generally, the mean success rate of attack modes used by non-territorial eagles was 0.54 ± 0.50. Floaters successfully obtained food through: kleptoparasitism (43.10%), carrion feeding (24.14%), and high soar with vertical stoop (14.66%). Several important issues for the conservation of non-territorial Eastern Imperial Eagles arose from our research. The strong relation of floaters with the European Souslik calls for specific conservation measures aimed at the conservation of this type of prey and the restoration and appropriate management of its grassland habitats. The importance of the scavenging behavior of juvenile birds requires increased control of the use of poison baits and subsequent prosecution by state institutions. Protecting the most important temporary areas, improving institutional control against the use of poison baits, and intensifying awareness-raising campaigns among pigeon-fanciers and hunters are also of crucial importance for effective species conservation.
... Many predators consume multiple prey species, and optimal foraging theory predicts that diet composition will change with (among other factors), prey abundance (Emlen 1966;Estabrook and Dunham 1976;MacArthur and Pianka 1966;Pulliam 1974). Though the strict "consume-always-ornot-at-all" behaviour often predicted by theory may not hold in nature for a variety of reasons (e.g., information constraints that prevent predators from having complete knowledge of prey abundance and distribution), the more general point, that diet will change with prey abundance, is borne out by numerous studies (e.g., Naef-Daenzer et al. 2000;Steenhof and Kochert 1988;Suryan et al. 2002). ...
Full-text available
Species interactions are thought to underlie the stability of ecosystems, and nowhere is studying such interactions more important than the rapidly changing Arctic. The foraging behaviour of generalist consumers is influenced by the abundance of multiple resources, and generalists are thought to confer stability to resource populations. Surprisingly, explicit treatment of the diverse prey communities that many predators encounter in nature has been relatively rare, with most studies confined to predator-prey pairs. My thesis investigates the relationships between predator and multiple prey in an Arctic ecosystem on the western coast of Hudson Bay from 2015-17, using Peregrine Falcons (Falco peregrinus) as a model species. First, I set out to quantify prey abundance on the landscape using distance sampling for avian species and Arctic ground squirrels (Urocitellus parryii), and a combination of burrow counts and snap trapping for microtine rodents (lemmings and voles). Results of snap trapping indicated 2015 was a year of low microtine abundance, while abundance was highest in 2016 and slightly less high in 2017. Burrow counts and distance sampling data were analyzed using density surface modelling according to six habitat covariates, and results indicated that freshwater, productive vegetation, and low elevation were the most consistent predictors of avian abundance across species and groups. Terrain ruggedness positively influenced abundance for Arctic ground squirrels and microtine rodents, while Arctic ground squirrels specifically were more abundant at low elevation, in areas with little freshwater, and in areas with productive vegetation. Conversely, microtine burrow counts were higher in areas with freshwater that were far from the coast. Second, I analyzed the abundance of the most common prey types for Peregrine Falcons in relation to distance from falcon nests to evaluate evidence for a “landscape of fear” that structured prey distribution. I found songbird and goose abundance to be positively related to distance from falcon nests, and in the case of songbirds, this relationship was present even during falcon incubation, when prey consumption is relatively low. This I argue, likely indicated avoidance of breeding Peregrine Falcons when songbirds arrived in the study area and established territories. Goose abundance was only lower near falcon nests in late summer, when vulnerable goslings entered the population. Unexpectedly, duck abundance was negatively influenced by distance from falcon nests in late summer, which I argue was likely due to similar nesting habitat selection between Peregrine Falcons and Common Eiders (Somateria mollissima), which were the dominant duck species detected in surveys. Finally, I used distribution maps constructed using the aforementioned density surface models to fit a complex multispecies functional response model utilizing nearly 11,000 prey deliveries recorded by remote cameras placed at Peregrine Falcon nests. Considering uncertainty in prey identification, camera failures, and prey abundance estimates, the resulting model demonstrated negative impacts of microtine rodent (lemming and vole) abundance and food supplementation (from a concurrent experiment) on the consumption of other prey. This indicated a potential short-term mutualism between prey types as falcon diet shifted with the microtine rodent cycle, adding to a large body of literature demonstrating the indirect effects of microtine rodents on other Arctic fauna. Model predictions indicated a wide range of biomass consumption across nests. Predictions with a random effect of nest site-year combination differed substantially from those without, indicating potentially strong individual differences in foraging between breeding pairs in this population. Predicted biomass consumption was most strongly related to the abundance of small birds (songbirds and shorebirds), indicating Peregrine Falcon nestlings may face an energy shortage at nests with low local small bird abundance. Surprisingly, biomass consumption by nestlings was generally unrelated to experimental food supplementation, providing context for a previous study demonstrating higher nestling survival at supplemented nests. Overall, my thesis provides insight into how Peregrine Falcons, as apex predators of the Arctic, provision their offspring and mediate indirect interactions among prey, and is a rare investigation of predator functional responses in a multi-prey context.
... A lower availability of preferred prey items (i.e. lepidopteran caterpillars; see Naef-Daenzer et al. 2000) in urban environments compared to non-urban ones corroborates the food limitation hypothesis further (Pollock et al. 2017; Seress et al. 2018; Baldan and Ouyang 2020). Notably, the nestling diet of tit species has been observed to contain signi cantly fewer caterpillars in urban birds compared to rural birds (Pollock et al. 2017; Jarrett et al. 2020). ...
Full-text available
Birds breeding in urban environments have lower reproductive output compared to rural conspecifics, most likely because of food limitation. However, which characteristics of urban environments may cause this deficiency is not clear. Here, we investigated how tree composition within urban territories of passerine birds is associated with breeding probability and reproductive success. We used seven years of data of breeding occupancy for blue and great tits ( Cyanistes caeruleus; Parus major ) and several reproductive traits for great tits, from 400 urban nest boxes located in five parks within the city of Malmö, Sweden. We found that tits, overall, were less likely to breed in territories dominated by either non-native trees or beech trees. Great tit chicks reared in territories dominated by non-native trees weighed significantly less, compared to territories with fewer non-native trees. An earlier onset of breeding correlated with increased chick weight in great tits. Increasing number of common oak trees ( Quercus robur ) was associated with delayed onset of breeding in great tits. Notably, as offspring survival probability generally increased by breeding earlier, in particular in oak-dominated territories, our results suggest that delayed onset of breeding induced by oak trees may be maladaptive and indicate a mis-match to this food source. Our results demonstrate that tree composition may have important consequences on breeding success of urban birds, but some of these effects are not consistent between years, highlighting the need to account for temporal effects to understand determinants of breeding success and inform optimal management in urban green spaces.
... Lastly, and as predicted, we found that both body mass and wing length of the chicks increased with the number of oaks in the territory. This is likely due to the well documented reliance of great tits on caterpillars from oak trees during chick rearing (Betts, 1955;Naef-Daenzer et al., 2000). What is more surprising, is the significant interaction effect between the number of non-native trees and artificial light sources on nestling body mass (Figure 5a). ...
Full-text available
There is a long history of avian studies investigating the impacts of urbanization. While differences in several life-history traits have been documented, either between urban and rural populations or across generalized urbanization gradients, a detailed understanding of which specific environmental variables cause these phenotypic differences is still lacking. Here, we quantified several local environmental variables coupled to urbanization (air pollution, tree composition, ambient temperature, and artificial light at night [ALAN]) within territories of breeding great tits (Parus major). We linked the environmental variables to physiological measures of the nestlings (circulating fatty acid composition [FA], antioxidant capacity and an oxidative damage marker [malondialdehyde; MDA]), to garner a mechanistic understanding of the impact of urbanization. We found that the antioxidant capacity of nestlings decreased with higher numbers of oak trees and levels of PM2.5 (airborne particulate matter with a diameter < 2.5 μm). Furthermore, the ratio of ω6:ω3 polyunsaturated FA, important for immune function, was positively correlated with PM2.5 concentration, while being negatively associated with ambient temperature and number of non-native trees in the territory. Body mass and wing length both increased with the number of local oak trees. We also show, through a principal component analysis, that while the environmental variables fall into an urbanization gradient, this gradient is insufficient to explain the observed physiological responses. Therefore, accounting for individual environmental variables in parallel, and thus allowing for interactions between these, is crucial to fully understand the urban ecosystem.
Full-text available
Global declines in bird and arthropod abundance highlights the importance of understanding the role of food limitation and arthropod community composition for the performance of insectivorous birds. In this study, we link data on nestling diet, arthropod availability and nesting performance for the Coastal Cactus Wren (Campylorhynchus brunneicapillus sandiegensis), an at-risk insectivorous bird native to coastal southern California and Baja Mexico. We used DNA metabarcoding to characterize nestling diets and monitored 8 bird territories over two years to assess the relationship between arthropod and vegetation community composition and bird reproductive success. We document a discordance between consumed prey and arthropod biomass within nesting territories, in which Diptera and Lepidoptera were the most frequently consumed prey taxa but were relatively rare in the environment. In contrast other Orders (e.g., Hemiptera, Hymenoptera)were abundant in the environment but were absent from nestling diets. Accordingly, variation in bird reproductive success among territories was positively related to the relative abundance of Lepidoptera (but not Diptera), which were most abundant on 2 shrub species (Eriogonum fasciculatum, Sambucus nigra) of the 9 habitat elements characterized (8 dominant plant species and bare ground). Bird reproductive success was in turn negatively related to two invasive arthropods whose abundance was not associated with preferred bird prey, but instead possibly acted through harassment (Linepithema humile; Argentine ants) and parasite transmission or low nutritional quality (Armadillidium vulgare; "pill-bug"). These results demonstrate how multiple aspects of arthropod community structure can influence bird performance through complementary mechanisms, and the importance of managing for arthropods in bird conservation efforts.
Full-text available
Species cross‐boundary response is a key mechanism affecting species spillover into agricultural fields. However, temporal changes in edge permeability, which may depend on the seasonal availability of resources in both matrix and native habitats, remain poorly understood. Here we tested how edge crossing behavior and the associated spillover of birds into sun coffee plantations respond to landscape structure and seasonality. We monitored the movement of six insectivorous understory bird species (four forest specialists and two forest generalists) using an automated telemetry system along a gradient of forest cover (7–60%) during two seasons (dry versus wet) at nine sampling sites at the Brazilian Atlantic forest. We monitored 116 individuals and obtained a total of 15 129 bird detections across seasons. Bird resistance to crossing edges was strongly driven by an interaction between seasonality and forest cover in the surrounding landscape, with higher resistance to crossing edges along the dry season and in landscapes with lower amount of forest cover. Furthermore, spillover patterns in plantations were driven by an interaction between forest cover and distance from forest edges, but this was most pronounced for forest‐specialist bird species. Forest specialists moved more intensively and farther from edges in more forested landscapes, whereas forest generalists showed similar patterns of occupation regardless of forest cover and isolation. Our study contributes to a better understanding of avian cross‐boundary responses and spillover in response to landscape structure across seasons, and the factors driving bird movement decisions in anthropogenic landscapes. There are a myriad of possible mechanisms governing movement decisions, and these mechanisms may interact in complex ways and remain important foci for research within the fields of tropical ecology and evolution.
(1) The aim of this paper is to estimate the shape of the curve relating first year survival to nestling weight in individual great tits (Parus major) and to study the causality of this relationship. (2) Data were collected in a mainland and an island population. Nestlings were weighed and sexed in the nest when 2 weeks old. A recapture programme provided data for recapture-rate estimates in the winter. Local survival until next breeding season was estimated by capturing the breeding population. Brood-size manipulation experiments were performed in the mainland population in order to manipulate nestling weights. (3) The relation between local recapture rate and nestling weight was described using logistic regression techniques. The descriptive model included positive weight and negative squared weight regression coefficients, if controlled for year, sex and date. Recapture rate approached zero at weights of c. 70% of the adult body weight. The curves for both populations showed an approximately linear part over a rather long range of weights. At high weights, the curve levelled off in the mainland population and curved down in the island population. (4) Survival from weighing till fledging and recapture rate from fledging till winter were related to nestling weight, but recapture rate from winter till breeding was not. (5) The effect of brood-size manipulation on nestling weight and subsequent recapture rate suggests causality of the recapture rate-nestling weight curve. Additional information from a comparison of the association between recapture rate and nestling weight within and between broods leads to the conclusion that weight does play a causal role in this relationship. Recapture rate-nestling weight curves can thus be estimated from non-experimental data.
1. Observations in Scots (Pinus sylvestris) and Corsican (P. nigra) pine plantations of Thetford Chase, Norfolk, during five breeding seasons 1952-56, describe the food, food stock and feeding habits of great tits (Parus major), blue tits (P. caeruleus), coal tits (P. ater) and willow tits (P. montanus). 2. The density of tits breeding in nest-boxes is recorded. Coal tits, adapted to conifers, were much the commonest species; great and blue tits, adapted to broadleaved woods, bred commonly in the pine only where there were nest-boxes; willow tits were scarce and rarely bred in nest-boxes. 3. The food of the young was recorded by direct observation and identifications were checked from meals collected with an artificial nestling gape. Numerically, caterpillars formed about 50% of the food of early and late broods combined. Caterpillars from Scots pine important for early broods were Evetria spp. (Eucosmidae) from buds and shoots, Thera obeliscata (Hydriomenidae) and Ellopia prosapiaria (Selidosemidae) from pine needles; all of which were virtually absent from Corsican pine. For late broods, caterpillars of Panolis griseovariegata (Caradrinidae) from both Scots and Corsican pine were much the most important prey. 4. Typically, the caterpillar stock in pine was low (especially in Corsican pine) in April and May, increased slightly in June, and reached a peak in late summer. This contrasted with broadleaved woods, where the stock was greatest in May-June when early broods of tits were in the nest. 5. Great, blue and even coal tits nesting in the pine collected much food for early broods, but little for late broods, from scattered broadleaved trees up to 400 m from their nests; and those in Corsican pine plantations also foraged in distant Scots pine. Caterpillars from sources other than pine formed the largest group in the diets of early broods of great and blue tits, but were less important for coal and willow tits. 6. Proportions of different prey species fed to the young varied greatly each year. The diet of early broods of great tits was characterized by the absence of Evetria larvae. Spiders and insects other than Lepidoptera were taken more often by coal tits than by great or blue tits. Large caterpillars, notably Panolis, were much more prevalent in the diet of late than of early broods of great tits; but the diets of early and late broods of coal tits were broadly alike (too few late broods of blue tits for comparison). The diet of great tits was always more varied than that of the other species, but especially so in the pine; that of blue tits was broadly similar in pine and broadleaved woods. 7. All tits selected larger prey than those available at random in the pine. Great tits selected larger prey than did blue tits, and blue tits than coal tits; and even when feeding on the same prey species on the same dates and in the same plantations as coal tits, great tits invariably selected the larger specimens. 8. In the pine, great tits breed on dates and lay clutches appropriate to the broadleaved woods to which they are adapted; but whereas in broadleaved woods early broods do better than late broods, because food is more abundant, the reverse happens in the pine where feeding improves later in the season. In the pine, early broods of great tits were short of food, but late broods not. 9. Each member of a late brood of coal tits received much more food than did each of an early brood, but this was not reflected in their weights: presumably neither early nor late broods were short of food. Though adapted to conifers, coal tits are more productive breeding in broadleaved woods. They remain scarce in broadleaved woods probably because unsuccessful in interspecific competition for food in winter. 10. The numbers of caterpillars eaten represented only a few per cent of those present. One exception was in Scots pine from 1 May to 15 June 1956, when coal tits alone ate some 48 000 Evetria larvae or pupae per 10 ha (25 acres), representing about 20% of the stock. Despite the usually low percentage predation, intraspecific competition for food in summer may account for the modest decline in average clutch size with increasing density of birds. In summer, tits are unlikely to eat a large part of their food stocks or perceptibly influence the numbers of their prey, because each prey species is available for only a short period. Percentage predation is likely higher in winter, but on a different array of species. 11. Nest-boxes influence the local distribution of, especially, great and blue tits in pine in summer, but probably do not increase their overall populations--determined by food shortage in winter. Together, pine and broadleaved trees provide a sustained food supply throughout the summer. Hence selected broadleaved trees interspersed through the pine plantations might both enable more tits to survive the winter and improve their breeding; and so in turn enhance the general level of bird predation on forest insects.
1. Fledging weight has been shown to correlate with survival in many bird species and, therefore, is an important component of fitness. Fledging weight results from growth during the nestling stage. Hence, environmental effects on nestling growth in altricial bird species play a key role in proximate explanations of selection after fledging. 2. We describe and test a method to quantify environmental effects on (daily) increments in nestling weight. It accounts for the known genetic component of growth and simultaneously detects the effects of short-term environmental fluctuations on growth. 3. The method is illustrated with data on daily weights of individual great tit nestlings from a rich and from a nutritionally marginal study area near Basel, Switzerland. However, the method can also be applied to other traits. 4. The observed weight increment is expressed as a ratio through division by the increment expected under good conditions. The expected increment is calculated from a Richards growth curve with a shape parameter derived from a fit on nestlings growing under favourable conditions and an asymptotic weight based on parental weight to incorporate the genetic component. 5. Under good conditions the ratio of realized over expected growth increment is on average close to 1.0 irrespective of nestling age. Under poor conditions, however, there is a decrease in ratio with an increase in age due to a larger influence of the environmental conditions later in the growth period. 6. The method is only useful when the expected growth increments are greater than the measurement error, in our examples about 0.1 g. 7. Monte Carlo simulations confirm that our interpretations are realistic. 8. We demonstrate that mean parental winter weight as an estimator of the asymptote is a practicable way of incorporating genetic variance in final body weight into growth models.
(1) This paper deals with the underlying mechanisms of the hunting behaviour and selection of prey species by great tits feeding their young at the nest, and is based on observations made mainly in mixed broad-leaved woodland in Wytham near Oxford, England, and partly in larch plantations in Yamanaka, Japan, using both automatic cameras (41 000 photographs taken) at ten nests for 150 nestling-days and direct observations at six nests for 63 nestling-days. (2) A brief description of general tendencies in nestlings' diet as observed by various authors in different parts of the world is given first, as an introduction to this study. The major prey species in the breeding season are invariably Lepidoptera, particularly the larvae. (3) Apart from spiders taken during the first few days after hatching, the composition of prey species in the diet of nestlings has no particular relation to their age. It is suggested that spiders are of particular importance, from a nutritional point of view, at a certain stage of the chicks' growth. The nestlings were regularly fed with grit and snail shell, whose function seems to be related solely to mechanical grinding of food in the gizzard. (4) The seasonal succession of the prey species in the nestlings' diet throughout the breeding season is described, and the collection sites for many of the prey species are discussed. (5) Factors governing the utilization of prey species by tits to feed their young are highly complicated and are discussed first in terms of observed facts. On the whole, there seems to be no direct correlation between the biomass of the prey in the habitat and the tits' selection. The occurrence, in the nestlings' diet, of many lepidopterous species coincided with their time of pupation, which suggests that the behaviour of the prey has some importance in relation to their exposure to predation by tits. The effects of taste and conspicuousness of the prey species and of alternative prey occupying different micro-habitats are discussed in relation to the hunting efficiency of tits. (6) The factors governing the utilization of prey by tits are discussed further with the aid of a theoretical model. Tinbergen's theory of search images is critically reviewed, and an alternative theory proposed. The model is based on one fundamental assumption: that the predator tries constantly to maximize its hunting efficiency within its limited ability to perceive the abundance of food in various parts of the hunting area. (7) The concept of `profitability', defined as the amount of food the predator can collect for a given amount of hunting effort, is introduced into the model, and the relationship between the profitability and the density of a given prey species is investigated. From this model, some conclusions are drawn, and tentatively tested against the available observations. As the profitability of a prey species is determined not only by the density but also by the size of the prey and the method of hunting of the predator, and as the predator tries to get the most out of the whole complex of prey populations in the habitat, a direct correlation between the numbers of a given prey taken by the tits and its density cannot be expected. Instead, it is shown that the tendency expected from this model fits, without apparent contradictions, both Tinbergen's observations and my own. (8) It is also shown that the size of prey has some bearing on the selection of food by tits and influences differences in the composition between the diets of adults, fledglings and nestlings. (9) Some suggestions are made in concluding remarks as to the aspects of the problem which need to be considered in future studies.
1. The possible competition for food between the great and blue tit (Parus major and P. caeruleus) in oak woodland, and their relationships with the coal and marsh tit (P. ater and P. palustris), was studied by means of gizzard analyses. There was little similarity in the diets of the great and blue tits, the great tit taking mainly adult insects, especially weevils, while the blue tit fed mainly on scale insects and small larval and pupal forms. The coal tit showed a preference for small free-living insects, and scales. The diet of the marsh tit consisted of adult insects, scales and some larval forms. Spiders formed a small proportion of the diet of all four species. The great and marsh tits fed more on seeds and nuts than did the other species, and oak bud and gall tissue were found in large quantities only in the blue tit. 2. An analysis of the insect food showed that the great tit was largely a ground feeder, except in the breeding season, while the blue tit fed mainly on the oak twigs, buds and leaves. The coal tit showed a preference for insects from trunks and branches, while the marsh tit fed both on the ground and in the canopy. The beaks of the tit-mice were adapted to the size and types of foods taken by the birds, enabling all four species to feed in the same places without necessarily competing for food. 3. Nestling foods of a brood of great tits and a brood of blue tits reflected the differences in the type and size of foods selected by the adults in the rest of the year. Caterpillars were taken by both species, but pupae extracted from curled leaves formed a much greater proportion of the diet of the blue tits than of the great tits. Adult insects, forming 19% of the great tit nestling food, were not taken for the young by the blue tit. Both species fed spiders, crushed snails and grit to the young. Most of the food items brought to the young blue tits measured under 10 mm, while most of those fed to the great tits measured over this length. 4. Measurements were made of the caterpillar and titmouse populations in a nest-boxed area and an adjoining unboxed control area in 1950 and 1951. The rate of predation by titmice on all defoliating caterpillars in the boxed area was about 1.4% in 1950, 4.8% in 1951, while in the control area it was 0.9% in 1950, 3.2% in 1951. The proportion of the winter moth (Operophtera brumata) larvae removed by the titmice in the two years in the nest-boxed area was 0.5% in 1950, 2.6% in 1951, and in the control area 0.3% in 1950, 1.7% in 1951. In both years the caterpillar population was relatively low compared with the 1949 populations. In the winter of 1950-51 it was estimated that the predation rate by titmice on the female winter moths might have been about 20% in both areas. This predation on the moth population may be affected by climatic conditions, and the abundance of alternative food for the birds.
The effects of sexual size dimorphism of parents, diversity of delivered prey items, and laying date on reproductive success were studied in a Great Tit Parus major population near Cracow, Poland: Although males were significantly larger than females, sexual dimorphism among pairs did not correlate with the diversity of prey delivered to young, nor with the number and average body mass of 14 days old young. However, the diversity of delivered food declined during the breeding season, while the average body mass of the young increased. Furthermore, the average body mass of young was higher if parents fed them with prey of lower diversity. Partial correlation analyses showed that this increase in body mass was not only a seasonal effect but was also related to the diversity of delivered prey items. The present findings do not confirm the intersexual niche differentiation hypothesis, even though the possibility for niche extension existed in the investigated population. Possible explanations for the negative relationship between fledgling weight and prey diversity are discussed.