Hudsonelster

Hudsonelster
Hudson's Elster ( Pica hudsonia ) in Denali National Park (Alaska)
Systematics
Order : Passerines (Passeriformes) Subordination : Songbirds (passeri) Superfamily : Corvoidea Family : Corvids (Corvidae) Genre : Real Magpies ( Pica ) Type : Hudsonelster
Scientific name
Pica hudsonia
( Sabine , 1823)

The Hudson's elster ( Pica hudsonia ) is a songbird species from the corvidae family . It is a typical 45–60 cm tall representative of the real magpies ( Pica ) with a long tail and shimmering, black and white plumage. The range of the Hudsonelster covers large parts of western and northern North America. There she inhabits open and semi-open landscapes with a moderate climate . Hudson's stars are omnivores . Invertebrates, fruits and carrion make up the majority of the diet, but their respective proportions vary depending on the season. The birds breed in monogamous pairs from March to June and build showy, spherical nests.

The Hudsonelster was first scientifically described in 1823 by Joseph Sabine . For a long time it was regarded as a subspecies of the externally similar magpie ( Pica pica ), from which it differs mainly in its singing and breeding behavior. Its closest relative is the Californian yellow-billed lizard ( Pica nuttalli ). The species population is considered stable, even if there are slight regional decreases. While the magpie had a neutral to positive reputation in the Indian cultures of North America, it was massively persecuted by the settlers from Europe into the 20th century.

features

Build and color

The Hudson's Elster is a medium-sized raven bird of about 45-60 cm total body length, close-fitting plumage, relatively short and rounded wings and a long tail. Only the longer tail, the somewhat longer wings and sometimes more blue iridescent feathers are supposed to distinguish them externally from the otherwise largely identical looking magpie. Unlike these, and as one of the few species in the Corvidae family, the Hudson's Elster has a distinct sexual dimorphism in terms of size and weight. Males grow on average 6–9% larger and 16–24% heavier than females. Males weigh 167–216 g and have a wing length of 205–219 mm. Their tail becomes 230-320 mm long, their beak measures 24.8-28.7 mm from the nostrils to the tip. In contrast, females weigh only 141–179 g. Their wings measure 175–210, their tail 232–300 mm. The female beak is between 23.0 and 26.6 mm long. The tail of the Hudson's elster consists of long, tiered feathers with straight ends. The middle pair of control feathers protrudes well beyond the rest of the tail, even more in adult males than in females and young birds. Although the individual measurements are usually not unambiguous identifying features, when combined they allow reliable identification of gender. The elongated beak is slightly curved. There is no clinical, i.e. gradual, variation in body dimensions across the range.

Drawing of the Hudsonelster by John James Audubon . The individual parts of the plumage of the species are easy to see here.

The plumage of the Hudson's magpies corresponds to the basic pattern of the real magpies . Nasal bristles , head, chest, upper abdomen and back as well as hand covers , control feathers, thighs, lower and upper tail covers are uniformly black in color and iridescent blue. The bases of the hair-like throat feathers are white, which is visible when the throat is distended. The shoulder coverts are white and form a distinctive white spot in the plumage on the wing. The lower abdomen and the flanks of the body plumage are also white. The arm covers and wings, the thumb wing as well as the outer flags, bases and tips of the hand wings are deep blue on the upper side and have a strong blue sheen. On the other hand, the underside of the corresponding areas are uniformly black and lackluster. The inner flags of the hand wings are white, which is visible in the wing as a narrow white stripe, in flight as a large white area. The tail is black on both sides. The middle pair of control springs and the outer flags of the remaining control springs have a metallic blue-green shine. Towards the end, the gloss changes to bronze-green, then to purple and finally back to blue-green. The legs are black, as are the inside and outside of the beak. The color of the iris varies over the year. In summer it turns light brown to gray-blue, and then takes on a dark brown tone towards winter. This development is repeated every year in adult animals. The nictitating membrane is light blue and has an orange spot.

This year's bird in profile. Characteristic for juvenile individuals are the bare areas on the face, the brown feather tips around the chest, the lack of shine and the loose plumage.

Young animals differ in color and morphology in some details from adult birds up to the first moult . Their plumage is less shiny than that of adult animals, the white areas look dirtier due to the sand-colored feathers, and the feathers are softer and less close. Since the black feather edges in the wing are wider than in adult individuals, the white areas on the wings are smaller overall. The tail feathers have round ends and are shorter than in adult birds, the outermost hand-wing is wider and less sickle-shaped; both characteristics persist until the second moult. In the first year of life, parts of the face can be feathered, but with the first moult the entire face is then feathered. The corner of the beak and the inside of the beak are pink; a white spot remains at the tip of the beak after losing the egg tooth . The nictitating membrane of juvenile birds does not yet have an orange spot. However, juvenile features (pink inside of the beak, bare areas on the face, white beak tip) can also reappear in some adult individuals, but these are usually not as pronounced as in young animals.

Flight image and locomotion

Striding Hudson Elster. The on tap passage reminiscent gait of birds will be replaced by hopping or short strides at higher speeds.

Thanks to its long tail, the Hudsonelster is an extremely agile and agile flier. It allows her to change direction abruptly in flight. In cross-country flights, however, the Hudsonelster flies at relatively low speeds, supported by powerful wing beats. She is able to fly straight ahead very slowly. In order to increase the flight speed, the birds do not increase the flapping frequency of the wings, but rather bring the body and tail more horizontally. J-shaped flight curves can often be observed in descent. The Hudsonelster puts its wings on strongly in order to fall 20–30 m and then to catch itself again. Horizontal obstacles such as hedges are only just flown over in order to dive behind them. When excited - for example in the presence of a predator - the Hudson's elster quickly spreads its wings two or three times in a row, which increases the signaling effect of the white hand wings.

Hudson stars are usually slow, but extremely agile fliers in an emergency

On the ground, the species usually walks in a brisk gait, with the body jerking forward. The tail is angled slightly so that it does not touch the ground. At higher speeds, the Hudson's Elster, like most other corvids, changes to a hopping gait, sometimes supported by flapping wings, which can also result in the departure of the birds. When excited, the Hudsonelster also hops sideways, for example to inspect an object.

Vocalizations

In their vocalizations, the black-billed magpie very similar to the yellow-billed magpie ( P. nuttalli ) and differs significantly from the outwardly more similar Elster ( P. pica ). The repertoire of adult birds consists of a series of rough, scraping calls, of which the cheeky staccato of the alarm call is particularly noticeable. The ka-ka-ka-ka-… often heralded by two slow skaa-skaa syllables can be found in a very similar form in the yellow-billed elster. In extremely threatening situations, it can also be replaced by a shrill scream and is only uttered in the event of a direct, mostly flying threat. In the case of a predator approaching on the ground, however, the Hudsonelster uses a slower smack. While the staccato alarm call is used for the immediate escape of other Hudson stars, the birds use the slower alarm call to call conspecifics to reinforce them.

Spreading and migrations

Recent distribution area of ​​the species. The brood distribution is largely based on the greater climate of the continent.

The distribution area of ​​the Hudsonelster is divided into a large area in the western center and a smaller area in the northwest of the continent. The southeast part of the area runs from the east of British Columbia (e.g. from Fraser-Fort George ) to the south parallel to the Pacific coast. In Canada it largely follows the course of the Rocky Mountains , while in the United States the Cascade Range and the Sierra Nevada roughly form the western border. The brood distribution extends in the south to Inyo County . From there, the dissemination border runs through southern Nevada , the northern Arizona and New Mexico and far west Oklahoma , north roughly along 100 ° W . The deposits are partially limited by the Missouri River . In North Dakota , the Artareal crosses the river approximately at the level of Bismarck and runs eastward into northwestern Minnesota . To the north it stretches from the Lake of the Woods over the lake district of the former Agassizsee and the southern half of Saskatchewan to the Athabasca and Peace Rivers , but lacks their upper reaches.

In the northwest, the occurrence extends - unlike further south - to the Pacific coast. It covers the outer west of Alaska up to the Alaska Peninsula , the Alaska range forms roughly the northern limit of the subarea. To the east, the distribution area extends into the southwestern Yukon and northwestern British Columbia, but no longer reaches the mainland coast and the offshore Alexander Archipelago in the southern part of Alaska . It is questionable whether the Hudsonelster also occurs regularly in the area between the two subareas. A closed distribution was previously assumed here, but the breeding records are extremely thin here, which is why the status of the species in this region remains unclear.

The current distribution of the Hudson Elster is attributed by ornithologists partly to climatic conditions, partly to anthropogenic influences. In the west the distribution correlates with the cold dry steppe climate , in the north it also advances into subboreal climates . In the east of the USA, high temperatures and precipitation (> 600 mm / year) may prevent further spread. Temperatures above 35 ° C are critical for the Hudson's elster - in contrast to the allopatric yellow-billed elster - and are not tolerated by them in the long term, as has been determined in laboratory studies. In the Pleistocene , its distribution extended to Florida , Virginia , Georgia and Alabama and also included the Texas Panhandle ; it was probably a common bird in the eastern part of the continent at that time. With the extinction of the bison ( Bison americanus ) in large parts of North America, the Hudson Elster also disappeared from these regions. The intensification of agriculture in Dakota, Nebraska and Kansas at the beginning of the 20th century probably resulted in the species being displaced further west. In the following decades, however, there was also a partial expansion of the spread. Minnesota was settled between 1930 and 1970, in Arizona the Hudson's egg has been a regular breeding bird again since 1970. Extensive clearing in Canada led to an expansion of the breeding areas to the north, in California the introduced common robinia ( Robinia pseudoacacia ) made it possible to repopulate the Owens Valley, which had previously been largely cleared . The transition from city centers to parks led to the colonization of urban areas in Canada in the 1960s and 1970s. In the western United States, urbanization of the species may have been prevented by introduced fox squirrels ( Sciurus niger ), which act as nest predators.

Hudson's stars tend to be resident birds that remain in their breeding area all year round, but there are certain migration tendencies . Young birds usually leave their parents' breeding area in autumn and then return to the next breeding season. Young bird populations observed in a study in Idaho migrated to higher mountain regions about 10 km from their parents' nest. In winter, especially in the north of the distribution area, there are high-altitude migrations, during which the birds move down into the valleys. In British Columbia, part of the population also migrates to the warmer Pacific coast in the south. An eruptive invasion of 1500 birds, as observed in Death Valley in 1919 , is an exception. The birds can cover large distances on these migrations: For example, a young bird from Saskatchewan covered 580 km south-east, and further record distances are 354 and 151 km. Hudson's stars also migrate irregularly beyond the borders of the breeding areas, but usually not further east than Minnesota. In the north the species has been sighted as far as Prudhoe Bay and Banks Island .

habitat

Hudson's eagle foraging on Cherry Creek Lake . During the breeding season, the birds are dependent on banks with sufficient bushes.

Especially during the breeding season, Hudson's stars are dependent on habitats rich in bushes along bodies of water. The surrounding landscape mostly has a mixture of open and semi-open habitats such as meadows, grasslands or pastureland interspersed with sagebrush ( Artemisia tridentata ). The Hudsonelster is far less demanding outside the breeding season. It then also penetrates anthropogenic habitats, especially those of an agricultural type. As a slow flyer, the species is primarily dependent on adequate cover from birds of prey and is therefore mostly found near bushes, woods and forest edges. Within cities, the Hudsonelster uses unspoilt riverbank areas as well as parks for breeding. Where human habitats are not available, it usually remains in the breeding habitats close to the shore. Forests and other closed habitats are avoided and only used as sleeping places.

Way of life

nutrition

A group of Hudson's stars on the carcass of a Canada goose ( Branta canadensis ). Carrion is an important source of food all year round and is particularly important during the winter months.

Hudson's stars are omnivores and feed mainly on insects, seeds and carrion. A large-scale analysis of the stomach contents of 569 American birds from the 1920s revealed a seasonally varying composition of the food spectrum. Between April and June, the diet consisted almost exclusively (92%) of animal material, mainly ground beetles (Carabidae spp.), Butterfly caterpillars (Lepidoptera spp.) And carrion. The main breeding season falls during this period, which means that there is a high demand for protein, while the supply of insects is high during this time. Between June and November there were mostly jumping horror (Orthoptera spp.), Fruits and carrion in the stomachs. In the winter months from November to March, the birds shifted more to vegetable food (grain and fruit), which made up about 60% of the food volume, supplemented by about 10% small mammals , which were mainly eaten in February and March, and between December and March February around 30% carrion. The composition of the food spectrum varies with the geographical latitude. In Alberta , vegetation studies found 92% of the diet in late February. These were mainly oats ( Avena spp.) And fruits of Prunus species. Only about 8% of the vaults contained the remains of human waste or remains from field mice ( Microtus spp.).

Bison ( Bison bison ) with a Hudson's elster on its back. The birds remove ticks from the animals' fur and thus secure food rich in protein.

Most of the food is consumed on the ground in an open environment. Like all corvids, Hudson's stars hide excess food in depots near the surface. Only one piece of food is placed per hiding place. In contrast to the predominantly seed-eating species, the hiding places are dug up again after one or two days. In the course of time, the animals move the originally randomly created hiding places in the direction of the nesting site. Hudson's stars actively observe each other looking for food and hiding and thus not only learn by watching, but also regularly clearing out other individuals' hiding places. Females in particular seem to have a tendency to exploit the hiding places of males: males dominate carcasses and similar sources of food, while females usually stand apart and later secure the pieces of food hidden by the male birds. Deer and cattle tolerate Hudson's star, while they clear the fur of ticks and other parasites . The most frequently eaten ticks include species of the genera Dermacentor , Rhipicephalus and Ixodes . Hudson's stars were observed killing animals up to the size of marmots ( Marmota sp.). These are usually weakened individuals with behavioral problems who are more likely to be captured in groups than alone. Compared to larger predators , such as the red fox ( Vulpes vulpes ) or the bald eagle ( Haliaeetus leucocephalus ), Hudson's stars often behave as kleptoparasites . The birds regularly form three to four inches (1.3–1.9 cm) bulges, which may be helped by swallowing the hair of deer and cattle.

Social and territorial behavior

A group of birds in the Garden of the Gods . Outside the breeding season, small swarms are the usual social unit in which Hudson's stars move.

Within the Corvidae family, Hudson's stars belong to the moderately sociable species. Young birds in particular come together to form larger flocks throughout the year, but adult individuals also join them in winter. More than 200 animals can then gather at shared sleeping areas. The formation of such large groups primarily has a social function in that it serves to form pairs, to learn through observation and to exchange information. The individual distance is never reduced at these sleeping places so much that the birds touch each other. Hudson stars also cooperate in hating potential nest robbers. Several breeding pairs come together to help the endangered pair in defense of danger. Unlike Eurasian magpies , Hudson's stars form stable linear, i.e. transitive hierarchies, with the beak length or the physical superiority of males as an indicator of rank. Outside of the breeding season, the usual social unit is a loose, small shoal that searches for food together. Low-ranking animals avoid the swarms, which are dominated by young males, and go foraging alone. If Hudsonelstern discover a dead conspecific, they start screaming loudly in order to attract more birds. These settle in the surrounding vegetation and join the screaming. Some birds fly down to the carcass, pace up and down and peck at the dead body before all birds leave the scene again after about 10-15 minutes.

Hudson's stars are less territorial than magpies. The breeding grounds are not defended all year round, which is why the swarming out of the young birds probably also has the purpose of occupying new territories. During the breeding season, the nesting site is defended against intruders within a radius of 30–50 m, the distance to the next nest is 55–300 m. In infraspecific disputes, physical attacks rarely occur due to the stable hierarchy. However, if a bird sees the hierarchy violated, contact fights can occur. Both Hudson stars then try to kick each other with their feet with a loud croak or to claw into one another, while fellow conspecifics bystanders accompany the scene with loud calls. If a bird wins, it stands on the inferior and picks it up with its beak. Often he is then distracted by an adjacent individual and involved in a mock battle so that the inferior conspecific can escape.

Reproduction and breeding

Hudson's stars form monogamous breeding pairs that are of different duration. They can last until the death of a partner, but the bond between the brood partners is often broken beforehand. Sexual maturity begins in the first year of life, but males usually only breed in the second year of life. The partners usually get to know each other in the winter swarm. Females tend to polyandry , which leads the male to attack potential competitors. The beginning of the breeding season depends on the available food. Abundant sources of food induce Hudson's stars to breed earlier than they would with a thin supply of food. Nest building begins in the south of the distribution area around January / February, in more northern latitudes later. Both partners participate in the construction. The female does most of the fine work, while the male takes care of the rough construction. The nest consists of a shell made of bulky, often thorny twigs, the inside of which is lined with mud, finer hair and plant fibers. A hood is attached over the nest, which closes the nest on all sides and gives it a high, spherical shape. This hood may serve to protect against American crows ( C. brachyrhynchos ), but it also increases the thermal insulation of the nest. The external dimensions are around 75 × 50 cm, but can also vary significantly. Since new nests are often built over old ones, the nest structures together can also reach a height of several meters. The nest hollow is on average 17 cm wide and 9 cm deep. The nest locations are very different and include the crowns of conifers as well as the branches of deciduous trees and shrubs, sometimes also power poles. It is usually placed at a height of 1–9 m, occasionally hanging over the water. Nest building takes one to three months.

After the nest-building is completed, the female lays 1–9, usually 6–7 eggs between the end of March and the beginning of June, which it incubates for around 18 days while the male takes care of it. The eggs measure 33.5 × 23.5 mm and are speckled with dark brown on an olive-brown background. The hatching young are fed by both parents and leave the nest after about a month. However, they remain near the nest for three to four weeks and are still dependent on their parents for six to eight weeks after they fledged. Usually three to four young fly out per clutch. Pairs with an older male are more successful in building nests, raise more young and start breeding earlier, whereas the age of the female has no influence on breeding success.

Diseases and Causes of Mortality

A major predator of the Hudson's magpie: the great horned owl ( Bubo virginianus )

As an omnivore, the Hudsonelster is the host of many different parasites , especially roundworms , suction worms and tapeworms . It is believed that almost all adult magpies are infested with endoparasites . The featherlings Docophorous communis and Myrsidea eurysternum are the most common ectoparasites. In addition, there are a number of ticks and flies that mainly affect nestlings.

In the first days of life, many nestlings, birds of prey , owls as well as ravens and crows also contribute to the death of many young birds. Goshawk ( Accipiter gentilis ), Great horned owl ( Bubo virginianus ) and large falcon species ( Falco spp.) Are among the main enemies of adult birds. In addition, many Hudson's stars are still victims of human persecution, especially in Canada.

Systematics and research history

A California yellow-billed catfish ( Pica nuttalli ). This species is the closest related to the Hudson's Elster.

The black-billed magpie was in 1823 by Joseph Sabine as " Corvus hudsonius " in the research report of the Coppermine Expedition John Franklin first described . The specific epithet hudsonia refers to the Hudson Bay , from where the type specimen of the description apparently came. Sabine erroneously assumed that both the Hudsonelster and the Elster ( P. pica ) lived in North America and that the Hudsonelster differ not only in their dimensions but also in a tuft of feathers on the back. For a long time it was regarded as conspecific with the very similar magpie before studies of its vocalizations and breeding habits by Tim Birkhead and Derek Goodwin suggested a closer relationship to the yellow-billed magpie ( P. nuttalli ). DNA tests finally confirmed this assumption. Birkhead also contributed significantly to the study of the behavior of the species, Charles H. Trost wrote a monograph for the Birds of North America .

Real Magpies ( Pica )   korean magpie ( Pica pica sericea )      Hudson's Elster ( Pica hudsonia )      Yellow-billed lobster ( Pica nuttalli )      Magpie ( pica pica )

Internal systematics of the real magpies according to Lee et al. 2003

The yellow-billed lizard, the sister species of the Hudson's lair, inhabits the warm, dry regions of California . Originally it was assumed that the yellow-billed magpies were displaced to the Pacific coast during the Pleistocene and displaced by immigrant magpies. The ancestor of both species, however, probably immigrated to North America about 1.5 million years ago via the Bering Strait and split up into the species that exist today 625,000–750,000 years ago. The species is monotypical , that is, no subspecies are recognized for the Hudson's Elster . Because it shows only a small genetic distance to other real magpies , some authors call for all species of the genus to be grouped under the species Pica pica .

Existence, settlement density and endangerment

The Hudsonelster population is considered safe. Bird counts as part of the annual Christmas Bird Count showed stable numbers, only in Nebraska and North Dakota there were stronger declines. The settlement density varies greatly depending on the breeding ground and food supply and is also influenced by human persecution. In southeast Idaho, for example, it is 35 nests per km², 10-16 nests in Utah and 2 nests in South Dakota . Regionally, breeding populations are threatened primarily by the decline and fragmentation of suitable bank habitats, stalking with poisonous bait and firearms as well as the disruption of breeding pairs, insecticides and the settlement of fox squirrels in their habitat. The Hudson's Elster, like many other species in the United States, was protected from persecution by the Migratory Bird Treaty Act of 1918, but persecution continues in Canada and the United States.

Cultural history

The Hudsonelster is part of stories and myths in all cultures in its area of ​​distribution, but is often only mentioned anecdotally in these. As with many other corvids, their reputation, their ability to learn and their curiosity played an important role. As a scavenger, she regularly followed bison hunters in the Great Plains and profited from the unused parts of the cattle killed. Their feathers and other parts of their bodies were used as jewelry by the inhabitants of the Great Plains, especially in warlike contexts. As a scavenger, it probably had a symbolic connection with the war. They were also notorious as forage robbers, when the Lewis and Clark expedition arrived on the Pacific coast, the participants found extremely fearless Hudson's stars that entered the tents to steal food. Usually they were perceived as friendly. The formation of the winter swarms was seen by the Tlingit as an important sign of the approaching cold season.

With the increasing persecution by the European settlers and the farmers of later generations, Hudsonelstern became shy; at the same time, they were mainly viewed as pests that endangered other bird species, could damage herds of cattle and spoil the harvest. This was accompanied by large-scale persecution and extermination campaigns, which further contributed to the negative image of the species and to the emergence of its shyness.

literature

• Tim Birkhead: The Magpies. The Ecology and Behavior of Black-Billed and Yellow-Billed Magpies. T & AD Poyser, London 1991. ISBN 978-1-4081-4024-6 .
• Steven D. Emslie: Avian Community, Climate, and Sea-Level Changes in the Plio-Pleistocene of the Florida Peninsula. In: Ornithological Monographs 50, 1998. pp. 1-113.
• Peter Enggist-Düblin, Tim Robert Birkhead: Differences in the Calls of European and North American Black-billed Magpies and Yellow-billed Magpies. In: Bioacoustics 4, 1992. pp. 185-194.
• Derek Goodwin: Crows of the World. 2nd Edition. The British Museum (Natural History) , London 1986. ISBN 0-565-00979-6 .
• Joseph del Hoyo, Andrew Elliot, David Christie (Eds.): Handbook of the Birds of the World. Volume 14: Bush-shrikes To Old World Sparrows. Lynx Edicions, Barcelona 2009. ISBN 978-84-96553-50-7 .
• Eugene S. Hunn, Thomas F. Thornton: Tlingit Birds: An Annotated List with a Statistical Comparative Analysis. In: Sonia Tidemann, Andrew Gosler (eds.): Ethno-ornithology. Birds, Indigenous Peoples, Cultures and Society. Earthscan, London and Washington, DC, 2010. ISBN 978-1-84407-783-0 .
• Edwin Richard Kalmbach: The Magpie in Relation to Agriculture. In: USDA Technical Bulletin 24, 1927. pp. 1-27. ( Full text )
• Gerhard Kooiker, Claudia Verena Buckow: The magpie. A raven bird in its sights. Aula Verlag, Wiesbaden 1999. ISBN 3-89104-633-2 .
• Sang-im Lee, Cynthia S. Parr, Youna Hwang, David P. Mindell, Jae C. Choea: Phylogeny of Magpies (Genus Pica) Inferred from mtDNA Data . In: Molecular Phylogenetics and Evolution 29, 2003. doi: 10.1016 / s1055-7903 (03) 00096-4 , pp. 250-257.
• Steve Madge , Hilary Burn: Crows & Jays. Princeton University Press, Princeton 1994, ISBN 0-691-08883-7 .
• Joseph Sabine: Zoological Appendix. In: John Franklin: Narrative of a Journey to the Shores of the Polar Sea, in the Years 1819, 20, 21, and 22. John Murray, London 1823. doi: 10.5962 / bhl.title.3631 , pp. 647-703 .

Web links

Commons : Hudsonelster  - collection of images, videos and audio files

• Charles H. Trost: Black-billed Magpie (Pica hudsonia) . In: A. Poole: The Birds of North America Online . Cornell Lab of Ornithology, Ithaca 1999. doi: 10.2173 / bna.389 .

Individual evidence

1. ↑ ^{a b} Sabine 1823 , pp. 671-672.
2. ↑ Madge & Burn 1994 , p. 120.
3. ↑ ^{a b c d e f g h i j k l m n o p q r s t u v w x} Trost 1999 . Retrieved March 5, 2012.
4. ↑ ^{a b} Madge & Burn 1994 , p. 121.
5. ↑ ^{a b} Enggist-Düblin & Birkhead 1992 , p. 185.
6. ↑ Emslie 1998 , p. 74.
7. ↑ Kalmbach 1927 , pp. 8–9.
8. ↑ Kooiker & Buckow 1999 , p. 94.
9. ↑ ^{a b c} del Hoyo et al. 2009 , p. 606.
10. ↑ Birkhead 1991 , p. 20.
11. ↑ Goodwin 1986 , p. 154.
12. ↑ Lee et al. 2003 , p. 255.
13. ↑ del Hoyo et al. 2009 , p. 505.
14. ↑ Lee et al. 2003 , p. 256.
15. ↑ Birkhead 1991 , p. 217.
16. ↑ Hunn & Thornton 2010 , pp. 199-200.

This article was added to the list of excellent articles on July 3, 2012 in this version .