Red-axed Cowbird
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Red-axed Cowbird ( Molothrus rufoaxillaris ) |
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| Molothrus rufoaxillaris | ||||||||||||
| Cassin , 1866 |
The Rotachsel-Kuhstärling ( Molothrus rufoaxillaris ) is a small songbirds from the family of Icterids . The species, which is widespread in southern South America , was first described scientifically in 1866 by the American ornithologist John Cassin . As with all representatives of the cowbird genus , the red-axed cowbird is a breeding parasite that leaves the rearing of its own young to other species. What is unusual here is the clear specialization in a single species, the brown cowbirdwhich, despite its misleading name, belongs to a different non-brut parasitic genus.
features
Red-axed cowbirds are rather small representatives of their genus with an average size of 19 cm and a wingspan between 105 and 124 mm. Males tend to be slightly larger and heavier than their female counterparts, their weight is around 63 g, while that of the females is around 50 g. However , there is no more extensive sexual dimorphism on the basis of which the sexes could be distinguished. The plumage of the red-axed cowbird shows a uniform black color almost all over its body, which also has a conspicuous, lilac-colored glossy effect under appropriate lighting conditions. Only on the axillary feathers is there a red-brown spot that gives it its name, which is not visible when the wings are put on. The silky-looking plumage should exude a characteristic, musky fragrance. The short, strong beak is gray like the legs, the iris of the eye is reddish brown. Confusion with males of the closely related blackbird ( Molothrus bonariensis ), which is very similar to the red-axed cowbird in terms of size and color, is very common.
behavior
The species is a resident bird that does not take part in the seasonal migrations. Individuals rarely leave their “home area”, which has a minimum size of around 15 to 25 hectares. In the past , this loyalty to the location was often interpreted as territorial behavior, but more recent research on banded birds has found strong overlaps in the home areas without any conflicts between individual red-axed blackbirds. Furthermore, the clearly recognizable tendency towards the formation of small to medium-sized swarms speaks against pronounced territorial behavior. An exception is fights between couples for access to host nests, which can be observed regularly during the breeding season. The flocks formed when eating and resting usually consist of around six to ten birds, and only in exceptional cases do groups of well over 30 individuals occur. Representatives of other species, such as brown and silk cowbird, regularly join in. In these mixed swarms, Red-axed Cowbird use the utterances of other species as an indicator of potential threats, but they themselves do not warn verbally when a danger is detected.
nutrition
Red-axed Cowbirds are generally omnivorous , but the main component of the diet is usually various seeds. The specialization in a seed-based diet varies from individual to individual, while only seeds and cereal grains were found in the stomachs of some birds, rice, flies, larvae, grasshoppers, moths and caterpillars were also found in others. During feeding, which takes place exclusively on the ground, the birds often form mixed flocks with brown cow and silky cowbird.
Reproduction
Red-axed Cowbird form monogamous pairs that stay together for at least one breeding season, but usually much longer. This behavior is unusual for a breeding parasite, since one of the main advantages of monogamy in birds is assumed to be greater breeding success through joint brood care. Why the species nonetheless reproduces monogamously is unclear, possible reasons include an easier finding of potential host nests, the protection of the partner or joint defense of one's own territory. The courtship is not very expensive and takes place comparatively rarely. The male presents his horizontally outstretched wings while shouting loudly and bows repeatedly to the female, in this position his head is stretched vertically upwards, the feathers of the hood are raised. These displays take place almost exclusively in trees and very rarely on the ground. The courtship behavior ends about a month before the actual breeding process begins, instead the male follows the female extremely persistently during this time and rarely moves more than two meters away from his partner.
Unlike other representatives of its genus, the red-axed cowbird has a very high specialization in a single host - the brown cowbird. Other species are parasitized only very rarely and in a localized manner. For example, there are regular reports from the north-west of Argentina, where the red-browed bundle-nested nest ( Phacellodomus rufifrons ), whose nests are extremely easy to spot, serves as an alternative host species. The spying out of brown cowbird nests often begins long before the start of the actual breeding season, which extends from October to March, depending on the host. Once a suitable nest has been selected, the female begins to lay eggs, which takes several days. In the morning hours only a single egg is laid per day until a maximum number of five eggs per clutch is reached. It is not known whether more than one clutch can occur in the same female during a breeding season. Spreading the eggs over several nests is common. It is just as common that a brown cowbird nest is parasitized by more than one red-axed cowbird. Nests with more than 20 foreign eggs have already been observed. The appearance of the eggs varies greatly within the species; their basic color can range from white, gray and pink to green tones. Furthermore, there are pits and spots in just as many shades of color. The average dimensions are around 23 × 18 mm and the weight is around 4 g. The eggs of the brown cowbird show similarly large variations in color, which can make the exact assignment in a parasitized nest difficult in some cases. The eggs of the red-axed cowbird, however, are usually somewhat more rounded, and they can also have broad, blackish lines that are not known from brownbird eggs.
The female has to coordinate her own egg-laying time with that of the host birds. If the eggs are laid too early, i.e. before the host's eggs, the host does not accept the eggs and pushes them out of the nest or completely abandons the nesting site, which regularly happens particularly in nests with a particularly large number of parasitic eggs. However, if the eggs are laid too late, the likelihood of a successful incubation decreases significantly. This means that the breeding success of the red-axed cowbird is rather low: For example, the Argentine ornithologist Rosendo Manuel Fraga found in a study in 1986 that only around 12% of the approximately 260 eggs observed were successfully hatched. However, when the young birds reach the nestling phase, more than two thirds of them also manage to fledge and leave the nest. In order to increase the modest breeding success, blackbird eggs found in the nest are regularly punctured by the red-axed cowbird parents.
The breeding period is around twelve days, young red-axed cowbird hatch on average only eight hours before the host's young. Immediately after hatching, they are still naked, their eyes only open after about two to three days. Their weight averages 3.4 g and their size is around 43 mm. This makes them somewhat larger and heavier than young brown cowbirds, which they otherwise look very similar to. The development proceeds quite quickly during the nestling phase, after about a week the first feathers appear, which quickly develop into a mouse-gray down dress. After 12 to 16 days, the nest is left, which sometimes happens before the ability to fly, which is only achieved after 13 days at the earliest. In these cases, the young birds are initially still running and climbing. Young red-axed cowbird imitate the begging call of the brown cowbird boys, but they are much more persistent and are not limited to begging their host parents. Young birds that are raised in captivity also try to beg food from approaching humans. After leaving the nest, the young birds remain completely dependent on their host parents for at least three weeks; a partial supply of food often takes place two weeks longer. Only after several moulting processes do clear differences in the appearance of the two species become visible. At this point in time, the young red-axed cowbird leave their host parents and join swarms of their conspecifics.
The parasitic reproductive behavior of the red-axed cowbird and its special association with the brownbird were first observed and described in 1874 by the Argentine-British ornithologist William Henry Hudson after years of unsuccessful searching for nests of the red- axed cowbird . He enthusiastically described his discovery as "like the discovery of a new planet".
Vocalizations
Red-axed cowbird are generally regarded as loud and vocal birds, which is reflected, among other things, in their common English name Screaming Cowbird (literally: "screaming cowbird "). Both sexes sing and shout equally. The chant should sound like a quickly and frequently repeating pe - tzeee , with the second syllable being drawn significantly longer than the first. In addition, there is a short call that sounds more or less like chuck , which is uttered primarily during the flight and in conflicts with other birds. The function of another call that sounds like a quiet plunk is still unclear. While some researchers like Herbert Friedmann interpreted this as an alarm call, later studies cast doubt on the existence of a dedicated alarm call in the red-axed cowbird.
Spread and endangerment
The red-axed cowbird inhabits more open areas such as grasslands and light forests, but also landscapes shaped by humans such as agricultural land and lightly built-up urban areas. Since the red-axed cowbird is usually dependent on populations of brown cowbird to raise its young, the range of the two species is almost completely congruent. It stretches over large parts of northeastern Argentina, Uruguay and almost all of Paraguay to the southern regions of Bolivia and Brazil. For some time now, the red- axed blackbird has been spreading increasingly in a north-easterly direction into the Brazilian state of Paraná and beyond, where brown cowbird has not yet been detected. Here the species evidently evades to other hosts for reproduction. Overall, the red-axed cowbird is common and not endangered. The IUCN therefore leads him to stand in 2018 under the status least concern , although for the way there are no current population figures. Among other things, because the Red-axed Cowbird benefits from human activities such as the clearing of forests, the organization assumes a sustained stable population development. Studies on the frequency of the two species in northern Argentina found a ratio of brown cowbird to red-axed cowbird of about 3.7: 1.
Systematics
John Cassin first described the red-axed cowbird in his 1866 article, A study of the Icteridae in Proceedings of the Academy of Natural Sciences of Philadelphia . The species is currently considered to be monotypic , geographical variations are also missing. The clear similarities in the appearance and behavior of the young birds led to speculation that the red-axed and brown cowbird could be closely related species. Genetic studies have refuted this assumption, however, more recently the great similarity of the young birds has been interpreted as a form of mimicry on the part of the red-axed cowbird to increase their own breeding success.
literature
- Catherine P. Ortega: Cowbirds and Other Brood Parasites . University of Arizona Press, Tucson, AZ 1998, ISBN 0-8165-1527-1 , pp. 64-98 .
Web links
- Recordings of vocalizations at xeno-canto.org
Individual evidence
- ↑ Ortega, pp. 65-66.
- ↑ Ortega, pp. 84-85.
- ↑ Ortega, pp. 86-87.
- ↑ a b Ortega, p. 83.
- ↑ Ortega, pp. 83-84.
- ↑ Ortega, p. 95.
- ↑ Ortega, pp. 88-90.
- ↑ Ortega, pp. 90-91.
- ↑ Ortega, p. 82.
- ↑ Ortega, p. 86.
- ↑ Ortega, pp. 66-69.
- ↑ Screaming Cowbird Molothrus rufoaxillaris. In: iucnredlist.org. 2018, accessed on August 18, 2020 .
- ^ Gunnar Hoy, Johann Ottow: Biological and Oological Studies of the Molothrine Cowbirds (Icteridae) of Argentina . In: The Auk . tape 81 , no. 2 , 2005, p. 186-203 , doi : 10.2307 / 4082768 .
- ↑ John Cassin: A study of the Icteridae . In: Proceedings of the Academy of Natural Sciences of Philadelphia . tape 18 , 1866, pp. 10-25 .
- ↑ Nick B. Davies: Cuckoos, Cowbirds and Other Cheats . Bloomsbury, London 2000, ISBN 978-1-4081-3666-9 , pp. 186 .