By Peter O'Donald
This booklet describes the result of a long term examine of the ecology, evolutionary genetics and sociobiology of a seabird, the Arctic Skua. This species is polymorphic: the birds express one in all 3, genetically varied different types of plumage - light, intermediate and darkish. The types differ in frequency from predominantly light within the north to darkish within the south. The examine was once undertaken with the purpose of explaining how ordinary and sexual choice act to keep up all 3 sorts of skua in its populations. the consequences exhibit that typical choice for light is balanced by way of sexual choice for intermediate and darkish. types derived from Darwin's theories of woman selection and sexual choice in monogamous birds healthy the breeding information of the Arctic Skua. Darwibn's perspectives on sexual choice are absolutely proven. The examine produced unique facts on breeding ecology, demography, inhabitants rules, sexual behaviour and territoriality. A bankruptcy on feeding ecology is additionally integrated within the booklet, which supplies an entire and principally unique account of the inhabitants ecology and sociobiology of a unmarried species of chook.
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Extra resources for The Arctic Skua: A study of the ecology and evolution of a seabird
Some birds are sexed simply by observing them copulate while we walk through the colony. Most birds can 18 1 The Arctic Skuas of Fair Isle be sexed by examination when they have been caught for ringing. The female, having laid the eggs, has the larger cloaca. This will be particularly obvious if an egg has recently been laid. The relative sizes of the cloacas of a pair of birds has been a very reliable method of determining sex. But the cloacal difference gradually disappears after the eggs have been laid.
The Stercorarius species in the Northern Hemisphere show more divergence than both Northern and Southern species of Catharacta. The Stercorarius species diverged in size: the Pomarine Skua is the largest and heaviest; the Long-tailed Skua is the smallest and lightest. Catharacta species are all similar in size. 1 gives the comparative measurements. On morphological grounds, the evolutionary relationships are easily deduced: Stercorarius mustfirsthave separated from Catharacta; the three species of 30 1 The Arctic Skuas of Fair Isle Stercorarius then diverged, the small Long-tailed Skua evolving farthest and diverging most from the ancestral Stercorarius species; finally Catharacta maccormicki diverged from C.
They have counted either the individual birds or the pairs. Counting pairs is the more reliable method, particularly in dense colonies where many birds are whirling about the sky in dog-fights. Has a particular bird been counted already, or not? It may often be impossible to say. If a pair has eggs or young chicks, they usually feign injury when intruders on their territory approach their nest or chicks. Different pairs can usually be distinguished from each other as an observer passes from one pair's territory to another's: the birds of one pair will be seen to feign injury, then the birds of the other pair.