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Family Cinclidae (Dippers).

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Abstract

We review the biology, ecology and behaviour of the World's five species in the genus Cinclus that together make up the family Cinclidae. Uniquely adapted among passerines for swimming and diving in fast-flowing, upland streams, we reveal contrasts and similarities across the species.
FINAL DRAFT FROM STEVE ORMEROD
May 2 2005
DIPPERS
Systematics
The Cinclidae comprises five Dipper species occurring in North and South America,
Eurasia, and NE Africa. They are extraordinary as the only passerines adapted to
exploit aquatic habitats fully by swimming and diving. Their similarity in shape to
large wrens led taxonomists in the past to locate the Cinclidae close to the
Troglodytidae. However, Dippers strongly resemble thrushes in general morphology
and the spotted juveniles of some species. Electrophoresis of egg-white proteins
confirms this affinity, and more recent DNA sequencing places the Dippers close to
both the Turdidae and Sturnidae in the proposed superfamily Turdoidea.
Among the Dipper species, global distribution and genetic evidence (mitochondrial
cytochrome b and ND2 genes) suggests with only minor uncertainty that the genus
Cinclus arose in Eurasia around four million years ago, but soon spread into the New
World. Phylogenetic interpretation indicates close affinities between the two old
world species (Brown and White-throated Dipper) and between the two South
American species (White-Capped and Rufous-throated Dipper), with these pairs
separated by the American Dipper. The new world and old world species are seen, in
turn, as two sister clades (see Fig 1).
Geographical progression from the old world into the new was accompanied by the
development of variations among the species – most obviously in plumage. Two
species are uniform, the American Dipper being pebble-grey and the Brown Dipper
chocolate-brown. The Rufous-throated Dipper is uniform grey-brown apart from a
chestnut throat. The other two species have varying amounts of white in the plumage.
This is confined to the breast in the White-throated or Eurasian Dipper except in one
form (called leucogaster) where the underparts are completely white. In the White-
capped Dipper of the central and northern Andes, the crown is white and two of the
three presumed races (leucocephala and rivularis) have a white throat and upper
breast. The third leuconotus has a white patch on the back and the underparts are
wholly white except for the lower flanks and vent. The South American species
(Rufous-throated and White-capped Dippers) both have a white wing bar which
becomes evident particularly when the wings are flicked. At present there is no firm
explanation for these intra-specific differences in colour pattern, although natural
selection for cryptic colouration coupled with features used by three species in inter-
or intra-specific signalling seems most likely. Elements of sexual selection are
possible given the use of some plumage features – such as the breast features – in
courtship (see below).
The juveniles in some of the Dippers differ in colour from adults, though this is not
universal. Juvenile American and Rufous-throated Dippers differ from the adults only
in being paler below with mottled underparts and they have a paler bill. Juvenile
White-throated Dippers, by contrast, are grey-brown above, much paler than the
adults, and have buffy underparts with darker speckling. They acquire the adult
plumage after about 6 weeks – or sometimes later - but still are distinguished until the
second spring by pale edges to the greater wing coverts. The juvenile Brown Dipper is
grey-brown above with white spots and the wing feathers and coverts are tipped
white. Feathers on the underparts are whitish tipped brown giving the bird a mottled
appearance.
The Rufous-throated Dipper is considered monospecific but the other four species
have been divided into races. These are often based on slight plumage differences, on
the characters of type specimens and on geographical distribution. However, initial
genetic evidence (mitochondrial cytochrome b) on the White-throated species
suggests the current intra-specific taxonomy could well be misleading and in need of
re-appraisal. Up to 20 races have been described in this species, differing mainly in
the colour of the head, nape, breast and belly, in the presence and width or otherwise
of a chestnut band between the dark belly and white breast, in the width of dark
feather fringes on the underparts, and size. Several of the presumed races are distinct,
notably the eastern leucogaster in which the belly is white. However, in central Asia
polymorphism occurs with all-dark birds living alongside white-chested birds.
Palaearctic races have a rufous-brown or blackish-brown breast and belly. Some
populations are uniform but others highly variable. The northern nominate race has a
blackish-brown belly and no chestnut band whereas gularis in Britain has a strong
chestnut area below the white breast, the chestnut extending onto the flanks and mid
belly. Juvenile gularis are also darker than juvenile cinclus. Other ‘races’ are less
distinct and may grade into each other and even in one country. In general, dark-
breasted forms as cinclus inhabit cooler wetter climates while rufous-brown forms - as
southern European and N African birds - are in warmer, drier areas. Males are
reported as darker than females but birds darken with age and there is much variation
in any one bird through the year because of feather wear. Size varies within
populations, but males are always larger than females. Ironically the race minor, first
described in North Africa on the basis of a small specimen, is poorly named.
Measurements of live White-throated Dippers caught in the Atlas Mountains in
Morocco proved to have exceptionally long wings and large weights (mean 76 g (9
males), 68 g (3 females) and this has been confirmed by more recent samples.
In the Andes, the three presumed sub-species of the White-capped Dipper are as
described above. In North America, the American Dipper is divided into five races.
One race has an extensive range down the whole length of the Rocky Mountains. The
other four, which differ only slightly in the shade of the plumage, occur in different
areas between Mexico and Panama. In Asia the three races of Brown Dipper vary in
the richness of the brown plumage.
Morphological Aspects
All dippers are dumpy thrush-like birds of 60-80g with a short cocked tail, males
being larger than females. They are highly adapted for an aquatic life-style, and their
size, strength and general hydro-dynamic shape is ideally suited for swimming and
diving in fast-flowing water. They also have soft, long plumage, a thick layer of down
and more body contour feathers than comparable passerines. They have a large oil-
gland at the base of the tail, the oil being used during preening for water-proofing.
The dense and well-oiled plumage enables dippers to survive in near-freezing water
and in the low temperatures found at high latitudes and altitudes. To thermo-regulate
at higher temperatures, dippers can stand in shallow water to increase heat loss
through the legs and feet, or use shade.
Dippers have short, broad wings with a powerful musculature, allowing use for
propulsion in water, often in torrential conditions. Ensuring that they lose this power
for only the briefest period during their annual moult, White-throated Dippers are
unusual among passerines in dropping all the inner primary feathers almost
simultaneously - in keeping with other aquatic birds. Their energy expenditure does
not increase at this time, but behavioural changes mean that flying and diving become
infrequent. Moult sequences are not well known among other Dipper species.
The Dippers’ legs are long, strong and equipped with sharp curved claws that grip
rocks and mosses on the river bed or banks. The nostrils are narrow and have a broad
membrane or nasal flap to enable closure during submergence. The bill is slender and
slightly hooked. The eyelids are covered with white feathers so that a flash of white is
evident when the bird blinks. Blinking always accompanies dipping, and is probably a
signal in courtship, threat displays and alert signalling to potential predators. White-
throated and American Dippers can blink 40-50 times per minute. The nictitating
membrane has variously been described as milk white or bluish white and as cloudy
or semi-transparent. This membrane may be moved across the eye independently or as
a dipper blinks. It is thought to have a protective function when the bird is underwater
and it may serve to remove water droplets or dirt from the eye. The eye itself has
enhanced powers of accommodation for vision in both air and water due to the well-
developed sphincter muscles of the iris which can change the curvature of the lens.
Eye-colour in adult White-throated Dippers is a rich chestnut but young birds have
eyes that are more olive-brown.
Habitat
All five species frequent upland fast-flowing streams and rivers, even to the most
torrential reaches of mountain ranges from the Rocky Mountains and Andes in the
New World to the Himalaya of Asia. Most mountain ranges in Eurasia are occupied
either by White-throated or Brown Dippers, or occasionally by both as in the Tien
Shan Mountains and the Himalaya. Although the White-throated Dipper occurs in NE
Africa in Morocco, Algeria and as a vagrant to Tunisia or Libya, no other dippers
occur on apparently suitable mountain rivers in south of the Sahara. Similarly, rivers
in the mountains of eastern North America are unoccupied. Dispersal across
extensive unsuitable habitat such as the Great Plains of North America or the Sahara
has clearly never been achieved.
Although most dippers occur in hill and mountain ranges up to altitudes of at least
5000 m, altitude itself is non-limiting and fast-flowing rivers even near sea level are
occupied. The basic requirements are well-oxygenated, clear and largely unpolluted
water where stony beds have abundant invertebrate prey, especially caddis fly larvae
(Trichoptera), stone-flies (Plecoptera), mayfly nymphs (Ephemeroptera) and calcium-
rich molluscs, crustacean or small fishes during breeding. Large numbers of salmonid
eggs – for example in rivers around the Pacific rim where there are large-numbers of
migrant salmon – can also promote large dipper densities. Birds need rocks from
which to feed or dive, and riffles, stony shoals and a stony or rocky substrate on
which to find prey. Suitable nest and roosting sites are important but often abundant
in this habitat type. Cliffs, moss-covered rocks and waterfalls, walls or bridges with
ledges or crevices all provide ideal nest and roost sites. Artificial structures such as
weirs, mill streams, walls and bridges can sometimes provide fast-flowing water and
nest sites at what would otherwise be the edge of the dippers’ range. White-throated
Dippers in Britain sometimes forage around the rocky edges of clear nutrient-poor
(oligotrophic) lakes and on sea shores, particularly outside the breeding season.
Watercourses in forests are favoured sites partly because trees provide a subsidy of
terrestrial insects to the river margins, notably caterpillars, but also because falling
leaf-litter is ultimately important for the production of stream insects. Birds will nest
on rivers in villages, towns and other urban sites, and tolerate more open watercourses
provided that the basic requirements are met.
Dippers have linear territories along rivers from around 300m to 2-3 km, with
variation depending on the area of feeding habitat and prey abundance. Productive
Dipper rivers have breeding territories that are contiguous but on marginal rivers there
may be unoccupied gaps of unsuitable habitat. Densities vary and recorded values are
0.7-10 pairs /10 km (Wales), 1.4-7.7 pairs/10 km (Scotland), 1.4-2.2 prs/10 km (SW
Norway), 2.4-2.7 prs/10 km (Germany), 4 prs/10 km (Harz Mts), 2.3 pairs/10 km
(Austria),. In mountains of the Khamar Dhaban in Russia, Dippers are scarce with a
density of only 0.4 birds per 10 km recorded. There may be 1-2 km between pairs but
on frozen sections none occur. At the other extreme, dippers can reach such high
densities – for example outside the breeding season along rivers in the Pacific north
west of North America – that territories break down and individual birds occur within
a few 10s of metres of each other.
The five species of dipper are mostly isolated geographically with the exception of a
zone of overlap in central Asia between the Brown and White-throated Dippers. In the
Himalaya, Brown Dippers are common from about 900 m to over 3,500 m,
occasionally over 5000m where streams remain unfrozen, while White-throated
Dippers occur from about 3500 to 4800 m, occasionally lower outside the breeding
season. The two species therefore overlap at higher altitudes in the Tien Shan
Mountains and on the northern Himalayan slopes. There is some suggestion, but little
quantitative evidence, that White-throated dippers occupy smaller streams than Brown
Dippers in these cases. The extraordinary richness of river birds in the Himalaya
means that ecological segregation from other abundant river passerines (e.g. Forktails
Enicurus spp., Plumebeous Water Redstart Rhyacornis fuliginosus, White-capped
Water Redstart Chaimorrnis leucocephalus) is far more of an issue, and this is
achieved by subtle partitioning of habitat use, prey size and diet.
General habits
In addition to diving, the most conspicuous habit of three of the five species of
dippers is the exaggerated bobbing of the body and at the same time, the flicking
down of the tail and blinking of the white upper eye lid. The two Andean species
appear to bob or dip infrequently but instead rapidly flick their wings to show a white
flash as they expose the white inner webs across parts of the primary feathers.
While other river birds dip, bob, wag their tails or flick their wings, this has often
been attributed to disguise behaviour that helps to make the bird less conspicuous
against the movement of fast-flowing water. In dippers, however, there is now clear
evidence that dipping or wagging is an inter-specific signal to predators that the bird
is fit and alerted to a predator’s presence. Dipping and wing-flicking in dippers are
also widely exhibited in courtship displays, territorial disputes and aggressive
behaviour towards conspecifics, probably as a signal of fitness.
Three dipper species commonly dive in shallow water (often < 1m), and can remain
on the river bed for up to 20 seconds. However, dives are mostly only three to four
seconds duration and often in series of around five per minute over five minutes or
more. How dippers can remain underwater while maintaining their position in
torrential water was a subject of controversy until filming in tanks showed
conclusively the behaviours involved. Rapid wing beats essentially keep Dippers in
position underwater, and allow forward movement. Although secondary, the legs used
in a running movement help propel the bird or to cling briefly to rocks on the bed.
When diving, dippers have a silvery appearance due air bubbles entrained in the
plumage. Dippers have many of the physiological adaptations for diving seen in other
aquatic birds. For example, there is an immediate drop in the heart rate as a bird dives
underwater, and further decline during a dive. On surfacing, the heart rate then
increases. Dipper blood has a higher concentration of haemoglobin than comparable
passerines allowing greater oxygen storage.
The flight of dippers is fast and direct, usually low over the water and invariably
following a watercourse. Dippers only fly higher when pushed to the edge of their
territory; they then double back often flying over the observer. During displays birds
may also fly high into the air. Dippers dispersing over mountain ridges, or on
migration, also gain more height than usual.
Dippers are territorial when breeding and often throughout the year. However, they
may associate together in extreme cold where rivers remain unfrozen or in
circumstances of unusually high food production. They also sometimes associate at
roosts. Although most Dippers roost singly or in pairs, in the autumn and winter some
gather at collective roost sites, often bridges. Good roosting bridges in the UK have
ledges, girders, crevices in stonework or drainage in which birds can perch. Roosts of
nine and ten birds can occur although birds seldom associate in close contact. Any
similar aggregations at natural sites are not known, with birds often more dispersed
for example in canyons, under boulders and, probably rarely, in trees.
Roosts increase in size in the winter when birds are no longer roosting near to nest
sites and when bridges have the thermal advantages of insulation from wind and cold.
Dippers are faithful to roost sites within and between seasons throughout their lives,
and fewer than 6% of birds studied in Wales changed roosts between years. Around
95% of birds fly <3 km, and often <1km to reach a roost. However, some fly as much
as 8 km implying knowledge of roost locations well beyond their territories. For some
birds, these movements will be the largest ever made in their entire lives.
Outside the White-throated dipper, few data exist on roosting behaviour, although the
American Dipper is known to use a similar range of sites alongside or over rivers.
Voice
All the dippers have a loud, musical, bubbling and wren-like song given by both sexes
throughout much of the year. Only when dippers are feeding young and moulting do
they cease singing. White-throated Dippers sing particularly vigorously in September
and October when they are establishing winter territories and again from January-
March onwards as they establish breeding territories. Dominant individuals probably
sing more. With its pitch, variety of notes, repeated short phrases and rather simple
repertoire, dipper song can often be heard above the sound of adjacent rushing water.
The song of the female can be distinguished by a series of whistles and being more
scratchy and less melodious or sweet than the male. Most observers agree that the
male is more vocal although some have claimed that both sexes sing equally.
The song may be given from a rock low in the water, from ice, from a fence post or
low stump, from the bank or on the wing. As members of a pair approach each other
they sing as they do in high display flights or pursuit flights. There is a sustained
sweet sub-song.
Contact ‘alarm’ calls of all five species are similar, and also are loud, high-pitched
and penetrating enough to be detected by conspecifics against perpetual river noise.
Calls are variously rendered as ‘zit’ or ‘clink’ so that the dippers’ generic name of
Cin-clus has onomatopoeic elements. The notes are repeated quickly two to four
times in succession whenever a bird is alarmed, apparently excited or signalling to
predators, and also when approaching its nest or when in flight. Flight calls may vary
slightly being a shrill metallic ‘zlint’ or high musical ‘zlink’. Other calls ‘zrik’ or
‘zerrrb’ have been described in stressful situations and a rattling call r-r-r-r or low
rolling zur-r-r-r greeting at nest between pair members and by the female when
soliciting, often near the nest. Other calls include ‘gri gri’ given by a bird emerging
from a roost and flying downriver and a low-pitched ‘go’, ‘zo’ or ‘kep’. Wing-
whirring has been reported and a rapid slapping sound heard during nest-building.
The young start calling in the nest from about 6-7 days old as adults approach the nest
and during begging. Their calls are shrill and squeaky, ‘zip zip zip’ or ‘zi zi zu zu zu’
and likened to the contact call of a Common Sandpiper Actitis hypoleucos. Begging
calls continue in post-fledged juveniles for some days until independent foraging
replaces provisioning by adults. Juveniles begin singing in their first autumn just
before completing moult.
Food and feeding
Upland streams support a rich diversity of aquatic invertebrates, and it is on this that
the five species of dipper largely depend. Small fishes or salmonid eggs can also be
locally important at some times of the year. Dippers spend 45-55% of daylight hours
foraging, although this proportion increases on less productive streams, during the
shorter days of winter and when provisioning young. Foraging behaviours include
wading in riffles or shallow water, probing among pebbles, leaf-turning, gleaning the
wetted surface of rocks, picking prey from vegetation and very occasional fly-
catching. The White-throated, American and Brown Dippers, famously, all dive and
swim for prey in deeper water, although diving has not been observed in the two
Andean species. Dippers deal with large items such as fish, cased caddis or large
dragonflies, by beating them on rocks before swallowing them. Diving and wade-
picking are the main foraging techniques of White-throated Dippers. Diving is more
common during the winter months when flows are higher. During major floods,
however, turbidity and turbulence appear to prevent effective or selective foraging,
and dippers then feed in smaller side-streams or in more terrestrial habitats.
Among all passerines, dietary studies on dippers have been some of the most
illuminating in avian ecology since prey are taken from a relatively narrow and easily
identified spectrum, food availability is easily assessed, and many potential contrasts
in behaviour are observable in the Dippers’ easily measured habitat. Prey remains
such as insect mouthparts and fish bones in regurgitates or faeces can be counted
accurately, and also used to reconstruct prey size, weight and hence selection original
size or weight of the prey. Prey remains in pellets and faeces are similar, with no
evidence found for larger prey being ejected in the regurgitated pellets. Best known
of all the species is the White-throated Dipper, in part following our own work in
Wales, but also following studies by other workers in Germany, Ireland, Spain,
Scandinavia, North Africa and elsewhere. Dietary trends have been quantified over
the year, on streams of different quality and over the nesting period in both adults and
chicks.
White-throated Dipper
Throughout the range of the White-throated Dipper the diet appears to be very similar.
Prey items are almost entirely of aquatic origin with the aquatic stages of insects most
numerous, but molluscs, crustaceans and fish supplementing the diet numerically.
Fish, in particular, and often small cottids (= sculpins or cottids), can make a
disproportionately large contribution to the diet by biomass because of their larger
size.
The annual energy requirements for a territorial pair of White-throated Dippers – not
dissimilar in any of the species based on body mass - ranges from 148,000 to 158,000
kJ yr-1 depending on whether one or two broods are reared. Allowing for assimilation
efficiency, these requirements represent around 10.5-11.0 kg dry mass of prey per
annum. In the breeding season two insect families dominate these requirements –
respectively mayfly (= Ephemeroptera) nymphs and caddis (= Trichoptera) larvae. On
base-poor streams however, where these insects are scarce, stonefly nymphs (=
Plecoptera) are the most numerous prey. Both nestling and adult diet changes
remarkably with age – providing evidence of very marked prey selection. Small
nestlings are fed mainly on Baetis mayflies but as the nestlings grow they are fed
increasingly on larger limnephilid and hydrosychid caddis larvae. Calculations
indicate clearly that any other strategy would be unable to deliver sufficient energy to
the nest, and the parents compensate by taking increasingly smaller prey for
themselves as their chicks grow. Unusual prey sometimes occur, for example moth
larvae such as Tortrix virida that fall into the riparian zone where oak trees overhang
the river.
During the moult period, immediately after birds stop breeding, and during the winter
months, White-throated Dippers are more opportunistic. While they still feed on
mayflies and caddis, they now take large numbers of smaller prey, including the
smaller mayfly families such as Baetidae, but also blackflies (= Simuliidae). Fish at
this time are important energetically, and in Wales the provide 5-6% of the prey items
but up to 65% of prey biomass. The bottom-dwelling Bullheads Cottus gobio
comprises 80% of fish prey. In some parts of Europe, freshwater shrimps Gammarus
are eaten in large numbers. Also, Dippers in coastal areas can periodically occupy the
high tide line amongst sea wracks. In a Norwegian fjord, birds apparently took marine
molluscs Littorina saxatilis and two species of Gammarus. In Scotland on the coast of
the Isle of Rhum Dipper pellets contained mostly Gammarus and Orchestia
amphipods. Fish and other calcium-rich prey such as crustaceans and molluscs may
be particularly important in the late winter and early spring – just as reserves of
calcium for egg-formation become crucial.
In addition to seasonal changes, short-term changes such as floods can also affect diet.
For example, during floods in southwest Ireland, diet during baseflow was dominated
as normal by caddis larvae, especially Limnephilidae. During storm-flow, however
smaller Baetidae and simuliid fly larvae. Diet during the spate period also contained
more terrestrial invertebrates – either because birds took to terrestrial foraging or
because terrestrial prey contribute increasingly to casual drift in rivers as rainfall leads
to dislodgement. Similar patterns occur elsewhere in Britain and may be general.
Work in Wales not only addressed diet, but also possible impacts on prey abundances.
Annual exploitation could reach around 1-2. 5 g dry mass m-2 per year which, for
some highly selected prey, could account for a considerable proportion of production.
Thus, for example, dippers annually probably take up to 0.25-0.3 g m-2 each of
Plecoptera or Ephemeroptera, but up to 0.8 g m-2 of fish and 1.2 g m-2 of Trichoptera .
Influences on density could be substantial according to available data on production
for cottid fishes, or hydropsychids and limnephilids caddis. So far, these
energetically-based estimates are untested in White-throated Dippers, but enclosure
experiments on American Dippers by Harvey and Marti indicate they may well be
real.
While many ornithologists concentrate on bird behaviour in foraging studies, prey
behaviour is also crucial. Jenkins & Ormerod (1996) investigated how behaviours in
prey altered their exposure to real or simulated foraging dippers, and for example
simuliid fliy larvae or hydropsychid caddis lacked effective escape and this well could
help explain the heavy predation incurred. Drift or movement in other families was
often delayed until bill contact – which could well be too late to avoid being eaten!
Brown dipper
Data on the foraging ecology of Brown Dippers are available from Japan (from
Eguchi) and from the Himalaya of Nepal and India (Buckton). In both locations,
Brown Dipper foraged for large prey by diving deeply among submerged rocks and
for small prey by wading and pecking in shallow water. Foraging by diving was
seldom observed from May to October, but increased from December to April, when
the Japanese birds bred. Echoing provisioning in White-throated Dippers, adult
dippers fed themselves mainly by wading (i.e. small prey), but fed nestlings by diving
(i.e. large prey). Specific prey were identified only in the Himalayan study, where
Ephemeroptera (mostly baetids), Trichoptera (mostly hydropsychids), Diptera,
Plecoptera and Coloeoptera, in that order, were the major prey.
American Dipper
Also an increasingly well-known species, American Dippers forage on many of the
same groups as the old world species. Mayflies (e.g. heptageniids) and caddis (several
cased families) dominate the diet in the breeding season and these groups plus fly
larvae, mostly chironomids, are eaten in the winter. These groups also appear to affect
distribution. However, in Pacific coastal rivers, seasonal food abundance in the form
of Pacific salmon eggs can contribute very substantially to prey intake. Some of these
eggs arise from the corpses of spent fish which spawn and die in very large numbers
after breeding.
Other species
Data on the diet of the other two dipper species is sparse. White-capped and Rufous-
throated Dippers appear also to feed largely on caddis, mayflies and blackfly larvae,
although in Ecuador White-capped Dippers sometimes foraged from vegetation on
river banks, apparently taking terrestrial prey as earthworms.
Breeding
Breeding biology is more fully known in the White-throated, American and Brown
Dippers than the two Andean species. Birds usually start breeding at one year
although some males may not breed until they are two to three years old.
Breeding systems
Most dippers are monogamous, the members of each pair defending a linear breeding
territory and helping with rearing. In some cases, birds occupy territory through the
year. While there may be some potential for re-assortment before breeding begins,
mate-fidelity between successive years and breeding attempts is strong. However,
there is one instance of itinerant breeding and mate change within a season in the
American dipper (Osborn). Some dippers are bigamous or polygynous, with males
helping to rear two, three or rarely four broods. Usually the associated females are in
contiguous territories, but we know of one case of poly-territoriality. The incidence of
polygyny varies greatly both within and between populations, generally occurring
where females outnumber males on a river. So far, polygyny has been reported in
White-throated Dippers in Germany, Wales and Scotland, and in American Dippers in
Colorado, though it may be more widedspread given that Dippers share some of the
key attributes of polygynous species (eg sexual dimorphism). In Colorado, polygyny
occurred in 4 of 31 birds, but laying dates varied from 14-40 days so that polygynous
males could sometimes provision more than one nest
In Wales, DNA fingerprinting confirmed one male as the father of two broods, and in
general polygynous males have greater breeding success than monogamous males.
The breeding success of primary females is not significantly different from that of
monogamous females, but secondary and tertiary females suffer a 25% reduction in
brood size probably because of reduced parental assistance from the male. Even
though females in polygynous pairings increase their provisioning, overall nest
feeding visits fall where females feed alone – for example where the time of chick
rearing prevents a male helping at more than one nest simultaneously.
Nest building
Nests are built by or over running water at a similar range of nest sites in all five
species. Sites include crevices in a cliff, cliff faces or ledges, behind waterfalls,
among roots in the bank or on or under rocks in mid river. Where sites are behind
waterfalls, the adults fly spectacularly through the curtain of falling water to reach the
nest. Artificial sites such as holes or crevices in stone or brickwork of walls and
bridges or girders or other ledges under bridges are readily used. Bridges and walls
provide relatively safe nest sites as such structures rarely collapse and rarely flood.
Where birds have a choice of a natural or artificial site they often choose the latter. In
Switzerland, bridge sites allowed birds to build earlier and lay eggs earlier than at
natural sites. Certainly on marginal lowland streams where natural sites are limited,
bridges enable dippers to breed successfully and to extend from their upland
strongholds. Occasionally nests may be in a fork of a tree, in a hole in a riverside tree,
on fallen branches over the river or in overhanging branches, and around 6% of nest
sites in Wales were in such sites. Nest boxes under bridges or on walls have been
utilised by White-throated and American Dippers and other Dipper species probably
would also. Very occasionally dippers nest away from water, even up to 100 m away,
in or on buildings.
In some territories, a Dipper pair may use two or three different nest sites in the same
or different seasons even when nests are successful. There is however, usually strong
fidelity to a particular nest site within a dipper territory from year to year. Often the
same nest, repaired and relined, is re-used for successive broods, or the birds may
build another nest alongside or adjacent to the remains of the first. Where similar nest
sites occur side by side as ledges and girders below bridges, a pair may start building
two or three different nests on adjacent ledges before deciding on the final choice. In
extreme cases they may finish two nests and lay in both. Nest sites can be used
traditionally for many years – over 30 - and in the White-throated Dippers in one
instance for 123 years. Individuals and pairs clearly do not live so long so that either
some intrinsic attribute of the location or “cultural” knowledge transferred between
successive members of breeding pairs must be involved.
Nest construction is similar in all species and reflects the intrinsic, evolved aspects of
building methods, the availability of similar materials in all upland river
environments, but more important the adaptive value of a warm, waterproof,
concealable design.. The nest is a large globular structure of moss and/or grass, with
an outer diameter of 30-45 cm. When the site is a cliff face or wall, the birds first
build a circular base for the back of the nest, sometimes adding mud to the moss to
better secure the nest. On ledges a large platform of moss may be formed on which
the rest of the nest is constructed upwards and outwards, resulting in a miniature
haystack. Where the nest is in a confined space as a crevice or in a nest box it may
lack the dome and sides, being a mossy base and inner cup. The cup is made of roots
and grass and is lined with leaves, occasionally with hair. In Russia feathers have
been recorded as the lining. In Europe dry Beech Fagus sylvatica or oak Quercus
leaves are the preferred material for nest lining by White-throated Dippers although
ivy Hedera helix, bramble Rubus, birch Betula and willow Salix leaves are also used
and occasionally dry bracken Pteridium aquilinum fronds. American Dippers have
been reported using cottonwood Populus tremula leaves and Ponderosa Pinus
ponderosa needles in the nest lining, and Rufous-throated Dippers as using Andean
Alder Alnus jorallensis leaves as well as morphologically similar pieces of paper and
plastic. Brown Dippers in Nepal used leaves of Himalayan Alders A. nepalensis. The
circular or elliptic entrance hole is low down at the front of the dome, and faces
downwards, almost invariably over running water. Nests of White-throated Dippers in
Britain and Europe are usually less than 3 m above the stream or river and often only
1-2 m high (60% of nests in Wales). They may be only a few cm above water on
stable trickles but rivers in flood in the spring rise and fall dramatically and nests need
to be above high floods. Nests under railway bridges and on buildings may be up to10
m above the river. In the Rocky Mountains nests of American Dippers are generally
higher on average than those of White-throated Dippers. In Colorado Price & Bock
considered that high inaccessible and wide ledges were the highest quality nest sites
because they were safer from predators and flooding, the latter being a major cause of
nest failure.
Both sexes can be involved in nest building, but the female usually completes the cup
and lining. Sometimes she alone does the building or the male may bring moss and
other material to the nest vicinity where the female collects it and incorporates it into
the nest. In the White-throated Dipper, the female or the pair builds whereas in the
American Dipper some authors report that the female builds the nest, the male taking
no or only a minor share of the work. Shaw found that the nest building in Britain
took an average of 28 days (25-32 days) but in Ireland Perry recorded periods of 10-
21 days. In Germany construction periods of 9-28 days (mean 18) and 5-45 days have
been reported, and in Switzerland periods of 6-39 days. Cold or wet weather may
extend the building period and nests built early in the year are formed over a longer
period than later ones. Where the breeding season is short as at high altitudes or when
the first nest has failed, building may be more rapid.
Time of breeding
In all species, the basic tendency is to breed very early in the spring – from July
onwards in the Andean species of southern hemisphere from January (some Brown
Dippers) or February onwards in the northern species. However, exact timings vary
between locations due to a range of ultimate influences and proximate cues. For
example, in far northern populations, short winter daylength and the resulting
requirements for migration in some populations (e.g. N. Scandinavia) delays breeding
by comparison with birds further south. River hydrological regimes – surprisingly
predictable in seasonal locations – also exert an effect. In temperate, maritime areas
such as Britain, breeding begins as the winter risk of high rainfall and flooding
declines from February or March onwards. By contrast, birds in Alpine environments
need to avoid floods generated by snowmelt, while Brown Dippers in the Himalaya
must avoid both snowmelt and subsequent monsoonal rainfall and probably breed
before both. Annual cycles of food abundance probably also have effects, and for
example the emergence of large benthic invertebrates as adults from spring onwards
will deplete rivers of key prey thus favouring early breeding. Finally, Dippers need to
fit other requirements such as moult into the annual cycle while conditions are still
favourable for easy feeding and this fact alone can truncate the breeding season.
Proximate effects include temperature and actual food abundance – with low
temperatures and reduced food abundance tending to delay breeding sometimes into
unfavourable periods. For example, at acidic sites in Wales and Scotland where
invertebrates and calcium-rich prey were scarce, birds laid significantly later than
birds at good quality sites with major consequences for post-fledging survival
(Ormerod & Tyler 1993).
The results of case studies reflect this array of features affecting time of breeding. In
Britain and Ireland, nest records from the British Trust for Ornithology show how
White-throated Dippers lay from late February to mid June with a peak in laying in
late March and early April. There is one record of birds on a clutch of eggs in the
second week of February. In south Wales, laying peaks in the first week of April, with
52% of 364 clutches started before 7 April. In a wider study area in mid and southeast
Wales, laying peaked in late March (1263 clutches 1978-1993) and a smaller but clear
second peak in early May. The breeding season on the Esk River system in Scotland
begins significantly later than in populations further south and west in the British
Isles. In Austria and Germany, the earliest clutches of White-throated Dippers are
laid in early to mid March but further north in Norway and Finland clutches are not
laid until April or early May in the south and in mid May further north. In parts of
Russia chicks from the first and only brood do not leave the nest until mid July. In
North America the American Dipper lays earlier at lower altitudes as do White-
throated Dippers in Wales. In Austria, Germany and the former Czechoslovakia,
White-throated Dippers lay 5-6 days later for every 100 m increase in altitude whilst
in the Swiss Alps they laid as much as 18-19 days later for each 100 m increase.
Climates, as we now know, are non-static due to local influences such as urbanisation,
to larger periodic variations (North Atlantic Oscillation; Pacific Oscillation) and to
progressive climate change. Any of these might affect Dippers. For example,
Scharlemann used museum egg collections to estimate laying dates from collecting
dates in White-throated Dippers. Although subject to some uncertainty, Dippers have
started laying progressively earlier over the past 150 years with breeding onset
correlated significantly with mean March temperature. Hegelbach investigated laying
along tributaries from 410 - 680 m asl flowing into lower Lake Zurich in Switzerland.
Dippers initiated breeding in February or March of each year but actual dates varied
from 9 February (1995, 1997) to 9 March (1992) among years. The median date for
the first 25 % of all broods advanced significantly with increasing February water
temperatures, with February air temperatures, and with the flowering of two early
plants (Coltsfoot Tussilago farfara and Wood Anemone Anemone nemorosa).
According to historical data, the initiation of reproduction for Dippers at Lake Zurich
now occurs earlier in the year than in previous records for Britain, Germany and
elsewhere in Switzerland. In this case, the strongest explanatory factor was increased
water temperatures due to urbanisation. Effects through accelerated life cycles of prey
must have been greater than direct climatic effects on Dipper condition.
Eggs, laying and incubation
The eggs of all Dipper species are white in keeping with many other species that used
domed nests or cavities. They are sub-elliptical, smooth and glossy. Eggs of the large
Brown Dipper are the heaviest and largest and those of the small Rufous-throated
Dipper the lightest. Shape varies within species and eggs of White-throated Dippers in
Wales, Scotland and Norway are thinner-shelled and of lower weight at acidic sites
than elsewhere probably because calcium-rich prey and invertebrates were scarce.
White-throated Dippers lay 3 to 6 eggs, and much less often 1,2 or 7 eggs, usually but
not always at one-day intervals. Clutches of 4 and 5 predominate as in Brown and
American Dippers. The few data available for Andean dippers suggest a smaller
clutch – with 2 eggs normal for Rufous-throated Dippers in Argentina and Bolivia. In
White-throated Dippers, mean clutch sizes from large study samples are 4.2-5 eggs
with slightly larger clutches in Poland, the Swiss Alps, some areas of Germany and
SW Norway (mean of 4.9-5.1 eggs) than in Britain in general (mean of 4.4 eggs), in
Wales (4.7-4.8 eggs) or SW Ireland (4.16 eggs). In Wales 67% of clutches were of 5
eggs and 24.3% of 4 eggs. First clutches started in March or April were significantly
larger than later clutches, mainly relays or second clutches. In Europe, seasonal
reduction in clutch size are common (Germany, Switzerland, Britain, Wales), but not
universal (Esk system, Lothian, Scotland). There is no clear altitudinal effect on
clutch size but birds at better quality sites lay larger clutches than those on poor
quality acidic sites in Wales, probably reflecting food scarcity and quality. In
Germany, birds on south-facing slopes produced larger clutches than on north-facing
slopes which Zang related to food supply. In Scotland, clutch size did not differ with
parental age.
In the species more extensively studied (White-throated, American and Brown
Dippers) the female alone incubates. Very occasionally males have been reported
incubating or covering eggs in the female’s absence. Females sometimes sit on eggs
before the clutch is completed, possibly incubating from the penultimate or earlier
eggs as asynchronous hatching has been occasionally noted in both White-throated
and American Dippers. Incubation is usually 16 or 17 days (range 12-18 days), but
may be longer at higher altitudes and in earlier clutches. Birds have sat for up to 42
days on infertile eggs before abandoning the clutch. During incubation the male may
feed the female on the nest and spends much time (40% or more) guarding the nest or
rather the incubating female against intruding males. Neighbouring males have been
reported removing eggs or young – a strategy that would increase his chances of
copulating with the female.
The chicks, brood size and brood period
As passerines, newly-hatched dippers are naked, incapable of regulating their body
temperature and totally dependent on both parents for food. At first the female broods
them almost continuously, only leaving the nest for a few minutes after 1-2 hours of
brooding. The male thus brings most food in these early stages, either feeding the
chicks directly or passing food to the female. Brooding periods decrease as the
nestlings grow, generally ceasing at 12-13 days. In monogamous nests, both parents
then play an equal share in provisioning the chicks. When the chicks are young adults
enter the nest to feed them. There is then is a carousel system of feeding whereby the
chick that has just been fed retreats (or is pushed) to the back of the nest. As the
chicks grow, they beg and call at the nest entrance where they reveal orange-yellow
gapes edged with white flanges. Chicks defecate outwards from the nest once big
enough (>9-10 days), or adults remove faecal sacs and drop them into the river.
Sometimes faecal sacs accumulate on rocks or in water below the nest and adults
sometimes dispose of fallen faecal sacs at least early in the nestling period. One pair
of Rufous-throated Dippers feeding well-feathered young (>18-20 days) was still
carrying away faecal sacs.
Brood size of White-throated Dippers, though slightly smaller than clutch size due to
a small number of egg failures or early mortalities within broods, generally follows
the same temporal and geographical trends. Broods are larger at higher latitudes and,
in Wales, broods were larger earlier in the season than later. This pattern has
sometimes been repeated in other European studies. Brood size is also affected by
territory quality, notably prey abundance: on acidic streams where prey is scarce,
broods are smaller than at circumneutral streams. In SW Ireland the mean brood size
at fledging was 3.48 per successful pair
The nestling period from hatching to fledging lasts for about three weeks or more.
White-throated Dippers remain in the nest in Britain for 20-24 days, on average 22
days, and there is a similar nestling period in Germany (23-27 days) and Switzerland
(mean 21.5 days). The nestling period for American Dippers is similar, being from
23-28 days whilst that of Brown Dippers is 21 days. Birds can leave the nest
prematurely if disturbed by a real or potential predator. They explode from the nest,
dropping or diving into the river where they expertly swim and dive. They soon
scramble out onto the bank and hide among rocks, vegetation or tree roots, where the
adults will continue to feed them. Some exploding youngsters must survive to have
allowed this strategy to evolve. The alternative – being eaten – clearly has no survival
value!
Dippers have a relatively high hatching and fledging success, presumably because the
nests are often inaccessible to many predators being over deep running water. The
domed structure also makes it harder for predators to see eggs or chicks than in open
cup nests, and the large mossy structures are often camouflaged. Hatching success in
the White-throated Dipper in British and European varies from 61%-76% and
fledging success from 81-94%. Overall breeding success, the percentage of nests that
produced at least one fledged young, ranges from 56%-68.4%. The mean number of
young produced per nesting attempt varies from 2-4. All these factors are variable,
annually and seasonally. In Wales and elsewhere, first broods are generally more
successful than second broods although in the Swiss Alps nests were more successful
as the season progressed due to the effects of flooding (snowmelt) or predators on
earlier nests. Younger chicks are safer from predators than older ones, presumably
because older chicks are noisy. Parental age of dippers studied in Scotland affected
breeding success, with adult pairs have greater success than pairs with at least one
first-year bird.
Subsequent broods
Having completed an early brood, the incidence of second broods is very variable. In
general, second birds require first attempts to have been completed early to allow
sufficient time for a further attempt. This possibility is clearly limited in later-
breeding birds at high latitudes, high altitudes or on poor quality streams and case
studies all bear out these trends. On high quality streams in the catchment of the
lower Wye in Wales, about 24% of pairs had second broods although in some years
on particularly good quality streams 80-90% might attempt a second brood. By
contrast, birds at sites on acidic rivers rarely made a second attempt. Similarly, in
Scotland, the incidence of second broods was much higher (60%) on the lower
reaches of the River South Esk which had abundant prey than on upriver stretches. In
Norway and Sweden where early breeding is probably limited by day-length, as few
as 5-6.5% of pairs had second broods. In Switzerland, the likelihood of second broods
was less at high altitudes >1300m, while at lower altitudes the incidence of second
broods varied from 3-38%. In Austria, Dippers at altitudes above 500 m also rarely
had a second brood. In other central European studies, 10-49% of pairs had second
broods. The duration of the breeding season and the lengthy incubation and fledging
periods makes a third brood almost, but not totally, impossible. In some instances
where three broods have been claimed, it is probable that one or more broods failed at
an early stage, encouraging the adults to make another attempt. In Germany however,
there are several records of pairs having three successful broods.
American Dippers are frequently double-brooded, and about 40% of pairs in a
celebrated Colorado study area had two broods although the number of pairs
attempting a second brood varied between years from 5.5%-70%. These figures are
however, inflated because Price & Bock included pairs which re-laid following first
nest failures as well as genuine double-brooded pairs. In Japan, Brown Dippers had
only one brood but in other parts of their range, they may well be double-brooded, as
may the two Andean species.
Post-fledging feeding, development and survival
Once fledged, young birds are fed for 1-2 weeks by both parents, sometimes the
parents splitting the brood to each tend one or two juveniles. Some authors have
found that females fed the juveniles more than males. However, where the female lays
a second clutch during this post-fledging period, the male may play a larger role.
Newly fledged Dippers sit quietly on rocks or pebble shoals near the nest, awaiting a
parent to bring food. As a parent approaches, the young bird moves towards it and
flutter or quiver their wings, calling loudly and begging. Those birds gaining more
food from begging overall gain more energy than those who attempt to feed
Sometimes they peck rather inexpertly at prey, especially small blackfly Simulium
larvae on rocks. If danger threatens, they dive or flutter away to hide among rocks or
tree roots. With increasing age, Dippers spend longer foraging independently, and
their movement, flying and foraging skills markedly improve. By the time they are 2-
3 weeks out of the nest they are able to use a variety of foraging techniques and
survive on their own. However, the time required to gain independence is variable
even within broods (Yoerg 1998). The young often remain within the parents’
territory until they have completed the post-juvenile moult at four to six weeks after
fledging. During this moult they replace body feathers and lesser, median and inner
greater wing coverts to more closely resemble
Post-fledging survival is particularly difficult to assess in any population study partly
because of the difficulty in retrapping sufficient survivors, but also because of the
difficulty in separating post-fledging survival from overall nest survival. In Wales, we
followed the fate of chicks from 743 broods in relation to brood size, time of hatching
and territory quality. For all brood sizes, post-fledging survival varies through the
breeding season, and most survivors are from attempts in the peak breeding period
implying that Dippers time their breeding correctly. This effect was so strong in our
study that in nearly exactly cancelled out the benefits in fitness of double-brooding;
this implies that the benefits to adults of breeding once in the optimum period and
twice at sub-optimum periods (i.e. very early and again later in the season) is finely
balanced. There is also evidence that the most common brood size in Welsh Dippers,
of four, is also the most productive, though this changes subtly through the season.
Territory quality, particularly the negative effects of acidity, probably has major
effects on survival by delaying breeding and hence fledging into breeding into periods
where post-fledging survival falls dramatically. The reduced weights of fledglings at
acid sites will also reduce survival chances.
Nest and post-fledging predators
True predators are notoriously difficult to assess unless events are witnessed directly,
and this is rare. Observers more often reconstruct events from putative clues, and
many records will also be biased towards causes that are more likely to be detected.
Small mammals, notably Brown Rats, appear to be one the main predators of eggs and
chicks of White-throated Dippers. In an analysis of British nest records Shaw
attributed 60% of nest failures to rats while Perry found that rats in Ireland caused up
to 75% of ‘natural’ nest failures. Feral American Mink in Britain and Europe are
probably also important predators but the scale of any effect is unknown. In Ireland
they were thought to be one of the major causes of mortality in young birds either just
before or just after fledging. Native European Mink also cause some nest losses as do
stoats, weasels and Otters. A Dormouse was reported taking a brood of chicks in
Poland. Some newly fledged Dippers fall prey to domestic, feral or wild cats Felis sp.
Some 43% of 77 British-ringed Dippers reported dead whose fate was known had
been killed by cats – but this included adult birds also. Corvids, notably Carrion
Crows Corvus corone, Jackdaws C. monedula, Jays Garrulus glandarius and Magpies
Pica pica are all known predators of White-throated Dipper eggs or chicks. Juveniles
and adults are vulnerable to predation by Sparrowhawks Accipiter nisus. Merlins
Falco columbaria, Kestrels F. tinnunculus and Peregrines F. peregrinus whilst Tawny
Owls Strix aluco occasionally take full-grown birds.
In North America predators of eggs or chicks of American Dippers include martens
Martes sp., skunks Mephitis mephitis and weasels Mustela frenata and Bushy-tailed
Woodrats Neotoma cinerea. Water snakes and even Brook Trout Salvelinus fontinalis
are known to have taken fledglings. Information on predators of other species is poor
although an Andean Pygmy Owl Glaucidium jardinii was suspected to have eaten a
female White-capped Dipper on its nest. Kovshar believed that Brown Dipper nests
were almost inaccessible to terrestrial and aerial predators.
Cuckoos Cuculus only very rarely parasitise nests of White-throated Dippers, but
Brown Dippers seem to be more commonly used as cuckoo hosts.
Movements
Being so well adapted to a wide temperature range, many Dippers throughout the
world are relatively sedentary over the annual cycle. This includes the winter period
providing that that there is sufficient daylight in which to feed and providing that they
can access food supplies where rivers remain unfrozen. However, true long-distance
migration is shown by White-throated Dippers breeding in the Urals and in
Fennoscandia where intolerably short winter days force movement. In this latter case,
birds fly south eastwards each winter into S. Finland, Latvia, Estonia, Denmark, The
Netherlands, Belgium and even Britain. Their annual cycle of weight gain is adapted
to accommodate this movement – for example in allowing sufficient gain in reserves
to return across the Baltic in spring. Other regular migratory movements can be made
more locally even within watersheds where some (but not all) birds vacate higher
altitude breeding territories in winter. The causes of variation in this strategy, the
dynamics and the consequences are still poorly understood, but the best documented
examples involve the American Dipper. In the Chilliwack watershed of British
Columbia, around 65% of birds are sedentary and 35% migrants. Residents in this
case at lower elevations gained the advantage of earlier nests initiation, a greater
chance of second broods, greater nest success and larger annual productivity.
Migrants from higher altitudes in this case study have very marked fidelity to
wintering locations. In the Front Range of Colorado, the proportion of birds vacating
higher altitude watersheds appeared to be large - perhaps over 70%. In the Russian
mountains of Khamar-Daban some White-throated Dippers migrate for the winter
down to more southern regions, while others remain on open river channels and
brooks that do not freeze. At the beginning of May birds move back into the
mountains, first concentrating at river mouths. Males then sing at the edge of ice or on
rocks. Brown and White-throated Dippers in the Himalaya are sometimes altitudinal
migrants, though others can still be found at over 4500m even in December and are
probably far less migratory than other river passerines in this region. In the UK
White-throated Dippers may move down to the coast in extreme weather.
Given that Dippers generally show high fidelity to breeding sites, the only other major
movements shown by Dippers during their lifetime occur during natal dispersal. In the
White-throated Dipper in Ireland and Britain, where populations are largely sedentary,
juveniles begin to leave their natal territory within weeks of fledging and movements
are largely complete by autumn (September-October). Following the well-established
pattern in other passerines, natal dispersal is sex biased with females (median 5-8 km
and up to c 45km) moving further than males (median 3-5km). For males, only
around 8-10% of birds cross watersheds in making such movements, while around 20-
25% of females cross one or more watersheds. Generally similar data on natal
dispersal are apparent from the American Dipper (mean distance c 18km), albeit from
smaller samples. Here, around 10% of all ringed individuals were subsequently
observed in different watersheds.
Birds captured and re-captured during ringing have allowed very detailed assessments
of survival and population dynamics with some of these studies classics in
ornithology (Price & Bock; Saether et al.). Linear territories, ease of capture,
recapture or re-sighting and the ease of recording environmental variation have also
meant that Dippers have been model species for the development of novel techniques
and methods (Bryant et al 1985, Lebretton. et al 1992). First year mortality of young
White-throated Dippers is 60-65% while annual adult mortality is 40-50% so that
some birds can live to around 8 years. Remarkably similar values were obtained by
Price & Bock for American Dippers of 38-52% annual adult mortality and 68-78%
first-year mortality. In White-throated Dippers, death in adults most often follows
breeding when body mass is lowest and the stress of rearing young probably apparent.
However, survival is variable in both space and time, and for example in northern
populations, reduced winter temperature reduces over-winter survival on both adults
and chicks. Saether and colleagues have used this remarkably robust observation from
4000 marked individuals to suggest how future climate warming might lead to
population increase. However, this effect cannot be generalised, and in more
temperate areas of Europe such as France and Britain, mortality increases in years
with marked floods or droughts so that future expressions of climate change across
large parts of Europe are more likely to be detrimental.
Parasites
Dippers probably carry a wide variety of internal parasites ranging widely between
acanthocephalans, cestodal tapeworms, cutaneous trematodes and trypanosomes.
Trematodes Laterotrema cincli have also been reported in Brown Dippers in Japan. In
Oregon, USA the stomachs of ten American Dippers contained 441 trematodes
identified as a new species L. cascadensis. In Bolivia, two new cestodes -
Cinclotaenia minuta and C. boliviensis (Cyclophyllidea: Dilepididae) were also
recorded from White-capped Dippers. New species in this genus - C. georgievi and
C. paradehiscens– have also been recorded in White-throated Dippers from the
Carpathians.
Ectoparasites are apparently uncommon in Dippers. Few louse-flies or hippoboscids
were found on a large sample of Welsh White-throated Dippers, but two widespread
species known from other birds were collected – Ornithomya avicularia and O.
chloropus, the latter being the first record on a dipper host. O. avicularia had already
been found on dippers in Germany where eight of 232 examined, were infested, each
with one fly.
Norther Fowl Mites Ornithonyssus sylviarium occasionally infested broods of Dippers
in Wales but the nestlings fledged successfully. The same species have also been
found on feathers and in the birds’ nostrils and in nests of Dippers in Germany where
in some instances and in some years infestation can be high among broods. One nest
of American Dipper contained about 400 Northern Fowl Mites.
Many other mites can occur in Dipper nests. The best examples are from seven nests
of the White-throated Dipper in Wales, where 54 mite species, from 46 genera, 27
families and 4 orders were found. The array of species included several pests of stored
food, species able to invade houses, species known to produce respiratory allergies,
and species usually found in moss or leaf litter. Examples included Tyrophagus
putrescentiae, T. longior, T. palmarum, Acarus farris, Dermatophagoides farinae and
Kleemannia plumigera. Red Mites which suck blood from the nasal cavity of many
species of bird, have also been found in Dippers.
Feather lice Philopterus cincli and Myrsidea franciscoli have been collected from
White-throated Dippers in Wales and in Germany and from American Dippers in
Colorado. Several fleas, the Moorhen Flea Dasypsyllus g. gallinulae and Hen Flea
Ceratophyllus gallinae were commonly found in nests of White-throated Dippers in
Wales. In the Swiss Alps, attacks on dipper nestlings by Moorhen Fleas were reported
to be a main cause of failures of first broods. Nests of White-capped Dippers can also
be infested with fleas as shown by a total of 592 fleas found in the nest of a second
brood. Other ectoparasites of dippers include blow-fly larvae Protocalliphora which
suck the blood of nestlings. Such larvae have been found on nestlings of American
Dippers in California, Utah and Colorado, very occasionally causing mortality.
The relatively low incidence of ectoparasites on dippers may reflect a lack of
systematic assessment, but the solitary and aquatic nature of dippers, and their
frequent preening and waterproofing, might also reduce incidence. Anting behaviour
has been recorded both White-throated and American Dippers.
Relationships with man
The charismatic underwater abilities of dippers, their special adaptation and flagship
studies of their ecology have combined to give Dippers a public image that is
overwhelmingly positive. Moreover, their widespread use of man-made structures as
nest sites – sometimes even within urban areas – means that contacts between people
and dippers are not uncommon. In most European countries, Dippers are now fully
protected but until the start of the twentieth century, they carried a bounty in countries
such as Germany and Scotland. It was perceived erroneously that they might affect
fish populations adversely, and on one Scottish estate alone, 548 White-throated
Dippers were killed between March 1831 and March 1834. Even today some
landowners will still not tolerate dippers on their waters. In North America, predation
by American Dippers on fish eggs have also led to persecution. However, impacts on
wild salmonid populations are unlikely because the fry and eggs taken are anyway
subject to major, natural mortality. At fish farms, effects can easily be offset by
protecting rearing ponds with netting. Finally, many of the fish eaten at least by
White-throated Dippers are from species other than salmonids.
In the Mediterranean and North Africa there is some deliberate persecution of White-
throated Dippers by man, either for sport or for food. The Berbers in Morocco
apparently regard them as having aphrodisiacal properties. Elsewhere there is little
evidence that man has any major impact on the Andean or Brown Dippers through
direct persecution, although indirect impacts through habitat modification and
pollution are more likely.
The role of Dippers as biological indicators has been increasingly highlighted since
work during the 1980s-1990s in Wales provided the first ever evidence that birds
could be affected by acid ran (Ormerod et al. 1985, 1986, 1988). Extensive work
showed that density, territory size, laying dates, clutch size, shell thickness, egg mass,
brood size, the incidence of second broods, total productivity, nestling growth and
adult condition were all markedly inferior on acidified streams than on better quality
(circumneutral) streams. Effects were large, and for example pairs began laying at
acid sites 13-22 days later than elsewhere, clutch and brood sizes were smaller at acid
sites by >0.5 eggs or chicks, and densities were often 10-50% of those on good
quality streams. Many of these results were subsequently replicated in other parts of
Europe, and new evidence showed the palisade layer of eggshells of eggs from
acidified areas were almost 11% thinner than in eggshells non-acidified areas.
Additional British evidence also showed that populations declined when streams
became more acidic and also that population distribution matched the expectation of
effects of acid deposition (Logie). The process involved also became clear. In what
are now some of the best known of all pollution impacts on streams and rivers,
acidification reduced the abundances of key prey types for Dippers including
mayflies, caddis larvae and, crucially for pre-breeding females, also calcium-rich prey
such as molluscs, shrimps and fishes. Blood calcium was significantly lower in both
males and females at acidic sites in the pre-breeding period than at circumneutral sites
while alkaline phosphatase – a major enzyme in calcium metabolism – was elevated.
Serum calcium was also lower in young nestlings at acidic sites, while bone growth
appeared retarded. Pre-breeding birds on acidic streams had to markedly increase
foraging time at the cost of resting and self-maintenance in order to maintain energy
gain. Finally, no other explanation was apparent for these effects – such as from
pesticides or PCBs. By 1995 – the last major survey of acidification effects on dippers
showed that populations had probably thinned out even further. More latterly – in the
middle of a new decade (2005) - there is now unequivocal evidence that acid
deposition over Europe has ameliorated considerably, and average river quality is
improving slowly. However, biological recovery has been partial, patchy and
probably offset over extensive areas by the continued effects of acid episodes. Full
recovery among Dippers to pre-acidification conditions is therefore still decades
away.
The realisation that all the Dippers had general value as indicator species – coupled
with measurements of contaminants in eggs made in conjunction with studies of
acidification – has led to much further research. Their value as indicators of a wide
range of contaminants, for example, reflects the position of Dippers as high-ranking
predators in river food webs. Moreover, observers can be relatively certain that any
substances contained in eggs or tissues must have been accumulated within a well
defined territory: contaminant sources have sometimes been pinpointed with
accuracy. More latterly, also, Dippers are being flagged as important indicators of the
physical quality of river habitats.
In Wales, failed and deserted eggs have been analyzed for well over 20 different
pollutants including mercury, DDT and its metabolites (DDE, TDE), lindane,
hexachlorbenzene, dieldrin and its metabolites (HEOD), and individual PCB
(polychlorinated biphenyl) congeners. In general, contaminant burdens are now
below those at which major biochemical or reproductive effects have been detected in
other passerines. Even though infertile eggs sometimes contain elevated
concentrations of some PCBs, the congeners found have relatively low toxicity (eg
congeners 138, 153, 180). Moreover, even at apparently contaminated sites, breeding
performance and post-fledging survival have been unaffected. Partly as a
consequence, White-throated Dippers have been particularly effective in revealing
variations in pollutant conditions between regions (eg Wales vs Ireland vs Scotland vs
Norway), between catchments in different land use (e.g. the effects of increasing
sheep density on dieldrin residues – formerly used in sheep dips) and even due to
individual point-sources. For example, one tributary of the Welsh River Severn, the
Afon Mule, not only contained the highest PCB concentrations ever recorded in
Dippers (0.49 (geometric mean)-1.29 (upper quartile) μg g-1 wet mass), but also a very
distinct signature of PCB congeners. Subsequent analysis of river water confirmed a
small dismantler’s yard as a source of contaminated drainage, although this same
source had never been revealed by routine chemical monitoring.
Researchers on American dippers have now started to use Dippers as bioindicators
and have assessed blood, egg and feather contaminants. Along the Arkansas river, for
example, blood from Dippers breeding on metal impacted streams was compared with
reference samples and found to have both higher lead concentrations and lower
concentrations of ALAD. This enzyme is involved normally in manufacturing a part
of the haemoglobin protein but it may also be strongly bound by lead. Lead
concentrations in birds, in turn, reflected concentrations in prey so that Dippers in this
case indicated food web contamination. In the Chilliwack Watershed in British
Columbia, Dippers have been used to assess contamination by both metals and
organochlorines. Concentrations of mercury, cadmium and copper in feathers, and of
total organochlorines, PCBs and mercury in the eggs of birds resident along the main
river were higher than in migrants that bred on tributaries but spent the winter on the
main river. In other words, contaminant profiles reflected the breeding site far more
strongly than the wintering site. Accompanying research using stable isotopes showed
how the birds contaminated by most of these compounds were taking a greater
proportion of Pacific Salmon eggs or fry (Oncorhynchus spp.). In other locations,
such contaminants are transported into river systems from the Pacific coastal shelf in
migrating salmon so that Dippers in these cases have a pivotal indicator role at the
interface of terrestrial and marine systems.
More traditional forms of pollution adversely affect Dippers too. In the Tiber river
system in Italy, for example, and in the Peak District of Northern England, White-
throated Dippers have been shown more likely to be absent from streams polluted by
organic waste and/or suspended solids. In the Italian case, birds had disappeared after
the quality of water had degraded indicating that population trends revealed water
quality. In Britain, many rivers are now recovering from past organic pollution, and
Dippers have slowly returned to former sites. At the same time, new threats are
arising for example from climatic change and agro-chemicals (e.g. in sheep dips).
Status and conservation
Four of the five Dipper species have relatively large ranges in which they are common
on suitable streams. Local declines have occurred from pollution or from the
degradation of riverine habitats – even in upland locations and on small streams often
considered the cleanest in the case of acidification. Of greatest concern is the Rufous-
throated Dipper because of its relatively small range and restricted population in
northern Argentina and southern Bolivia. It was listed as a Red Data species of global
concern in 1992 and the recent Threatened Birds of the World listed it as vulnerable
because of its small fragmented range and declining populations. No sub-population is
thought to be larger than 1000 individuals. There is little evidence for any recent
declines in Bolivia where the species occurs on rivers draining east-facing slopes of
two ridges of the Andean foothills in the province of Tarija. On the western of the two
ridges dippers occurred at relatively low densities (pair per 1-2 km) on the Rio
Calama, Erquis, Coimata and Victoria, tributaries of the Rio Guidalquivir and Rio
Camacho. Possibly up to 200 pairs occur on the western ridge. On the better-forested
rivers on the eastern ridge birds are more numerous. S. Mayer found Dippers in the
Tariquia Reserve south of Narvaez on the Rio Escalera, Achirales and Lorayo and
estimated 250-500 pairs on rivers between Entre Rios and the Argentinian border.
Rufous-throated Dippers occur too, less commonly, to the north of Entre Rios and
Narvaez as on the Rio Villa where pairs occur on the upper reaches at 350-500 m
intervals, and possibly sparsely in the province of Chuquisaca, to the north of Tarija,
where the species was discovered in the mid 1990s.
Rufous-throated Dippers in northern Argentina are thought to be declining due to
changes in water management and deforestation. Threats include river pollution from
mining effluent and nutrient enrichment, deforestation and consequent siltation of
river bed and removal of water from upland rivers for the irrigation of tobacco,
sugarcane and other crops. There is also possible competition with introduced trout
Salmo spp. for invertebrate prey. The construction of dams and hydro-electric
schemes are additional threats. On some rivers in northern Argentina birds do
however, still occur at high densities (pair per 400-500 m).
The ranges of some subspecies of American and White-throated Dippers are small
and these could be vulnerable to extinction. In Europe Tucker & Heath gave the
status of White-throated Dipper as secure but only provisionally because data were
lacking from many areas. They estimated that 56% of countries had poor data on
population sizes and 23% had poor data on population trends. In Hungary there has
been a decline and possible local declines elsewhere. For example, birds had
apparently disappeared from parts of Ireland, west Wales, SW and NE England and
parts of Scotland such as Galloway between 1968-72 and 1988-91. These areas have
acidic streams, many draining catchments planted with conifers. On islands in
particular, dippers might have suffered. The race olympicus of the White-throated
Dipper on Cyprus became extinct soon after 1945. Before the 1950s Dippers bred on
the Orkneys and Isle of Man; they then disappeared but had shown a slight recovery
by 1988-91. Dippers are a rare breeding species of the north-western regions of
Karelia in NW Russia although are more widespread during the periods of passage
and in the winter. Numbers of Dippers nesting in Karelia have considerably
decreased, but that may have happened before the 1930s/1940s. There are about 300
pairs nesting in near-polar regions of Finland where there is a trend for decrease.
Pollution from industrial effluents has caused declines of White-throated Dippers in
Britain, Poland, eastern Germany and possibly elsewhere across Europe. Dams and
hydro-electric schemes remove or alter stretches of river used by Dippers whilst river
training, modification by dredging or straightening might have adverse effects by
reducing habitat diversity. Irrigation and abstraction might reduce flows while
alterations resulting in extreme drought or flooding may also have consequences for
Dippers. Mining in the Andes causes both pollution and siltation and is believed to
have adversely affected White-capped and Rufous-throated Dippers. M. Sara has
reported declines of dippers on Sicily because of modification of rivers including
concreting of channels. There is also considerable speculation about the possible
effects of climate change which will change river flow regimes considerably. In many
of these instances, the exact effects could possibly be substantial but are poorly
quantified and only partially understood.
Catchment land use can affect river systems, for example by influencing erosion and
river hydrology. Across a large extent of the Himalaya, Manel et al.( 2000) showed
how stream habitat structure, chemistry, aquatic invertebrate abundance and the
occurrence of river birds were all affected by land use after accounting for altitudinal
pattern. Streams draining terraced catchments were particularly affected with adverse
consequences for Brown Dippers.
The replacement of old stone bridges with modern concrete box structures for heavy
traffic has resulted in the loss of many traditional dipper nest sites in some areas of the
UK. Ledges, drainage holes and nest boxes in these cases could also be simple to
install with benefit. Boxes have been used widely in Britain, Hungary, Austria,
Germany and elsewhere to try to halt the decline in dipper populalations when old
bridges have been destroyed. Rockenbauch found that more than a third of dipper
broods on 420 km of river over a 28 year period were in nest-boxes, whilst Kaiser
found 64% of nests in nest-boxes in his study area.
Perhaps a more surprising and alarming problem in view of all these potential effects
is that many national monitoring schemes for birds often under-represent rivers and
river birds. Others cover only a vanishingly small percentage of total river extent or
depend on a small numbers of enthusiasts. In many cases, therefore, changes in
Dipper populations could go undetected and their clear value as wider environmental
indicators may well be wasted.
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Figure 1 The most likely phylogenetic relationships among Dippers (After Voelker 2002)
Brown
Dipper
White-throated
Dipper
American
Dipper
Rufous-throated
Dipper
White-capped
Dipper
... Esta familia agrupa a cinco especies de pequeñas aves (de 14 a 23 cm) todas pertenecientes al género Cinclus, que se distribuyen en América, Europa y Asia (85). Todas las especies están altamente especializadas en la captura de insectos acuáticos en torrentes de agua limpia y oxigenada que normalmente se restringen a ambientes montanos, su método de captura consiste en buscar las larvas de insectos acuáticos removiendo las piedrecillas de las orillas, en ocasiones atrapando insectos adultos al vuelo, e incluso son capaces de sumergirse y bucear. ...
... Dos especies habitan en Sudamérica, y una de ellas, el Mirlo-acuático Coroniblanco (Cinclus leucocephalus) ha sido citada para el valle de La Paz (15); los autores mencionan que ha sido observada en lagunas y ríos sobre los 4000 m en la cuenca alta de Cota Cota (localidad de Tapacaya). Esta especie se distribuye sobre los Andes, desde Venezuela hasta el centro de Bolivia, principalmente en las cabeceras de ríos yungueños donde el agua torrentosa es cristalina; son principalmente residentes aunque pueden realizar movimientos altitudinales postreproductivos (85). Su presencia en el valle de La Paz necesita ser confirmada. ...
... They are widespread (Voelker, 2002) and are unique as they are the only passerines adapted to make use of aquatic habitats by swimming and diving. They usually require fast-flowing and well-oxygenated waters where stony beds especially offer abundant invertebrate preys such as caddis-fly larvae (Trichoptera), stoneflies (Plecoptera), mayfly nymphs (Ephemeroptera) but also calcium-rich molluscs, crustacean or small fishes during the prebreeding stage (Ormerod and Tyler, 2005). They are thus considered good indicators of changes in water quality (Ormerod and Tyler, 1993). ...
... Among the five species known in the world (Voelker, 2002;Lauga et al., 2005), the White-throated Dipper (Cinclus cinclus) has the widest distribution occurring in Europe, North Africa and Asia. Its ecology has been extensively considered (Spitznagel, 1985;Tyler and Ormerod, 1994) and researchers still exploit the ease with which it can be studied to develop both influential and novel research lines (Ormerod and Tyler, 2005). In particular, the seminal studies on activity pattern and energy use by David Bryant et al. (Bryant, Hails and Prys-Jones, 1985;Bryant and Tatner, 1988;Bryant and Newton, 1994;Bryant and Newton, 1996;Brown and Bryant, 1996) are of considerable importance. ...
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... we use that value as the threshold probability) into a binary map of suitable versus unsuitable habitat to create a map of the species' potential distribution (McFarland et al. 2013). Given that the Rufous-throated Dipper is restricted to fast-flowing, rocky mountain, forested rivers (Ormerod and Tyler 2005), we refined the species distribution map by including only pixels that overlapped with rivers. We used the river layer available from the governmental Geographic Information System datasets including rivers classified as permanent (i.e. ...
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The Rufous-throated Dipper Cinclus schulzi is endemic to the Southern Yungas of north-western Argentina and southern Bolivia. The species is categorised as ‘Vulnerable’ on the IUCN Red List on the basis of small population size and restricted range. The purpose of our study was to determine the distribution of potentially suitable habitat for the Rufous-throated Dipper, estimate its population size, and assess potential distribution within strict protected areas, in north-western Argentina. We surveyed 44 rivers in the Southern Yungas of Argentina from 2010 to 2013 to determine dipper density (i.e. the number of individuals detected per km surveyed). The dipper’s potential distribution was assessed using a maximum entropy modeling approach based on 31 occurrence points and eight bioclimatic and two topographic variables as predictors. The species is dependent on mountain forest rivers, so the potential distribution was restricted to rivers. We estimated dipper population size by multiplying density by the potential distribution along rivers. Finally, we calculated the extent of suitable habitat contained within the boundaries of Argentina´s National Parks. Dipper density was 0.94 ± 1.55 individuals/km. We estimate that within north-west Argentina there are ~2,815 km of river that are potential habitat, with an area of occupancy of 141 km ² and a population size of 2,657 ± 4,355 dippers. However, of this river extent, less than 5% is within National Parks. Our results highlight the need to create new and to enlarge existing National Parks that protect the potentially suitable habitat of the species. Although more information is needed for Bolivia, the country-level area of occupancy and population size of the dipper found in Argentina provides strong evidence that the IUCN Red List classification of this species as ‘Vulnerable’ is warranted.
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