Sexual selection in birds

Sexual selection in birds concerns how birds have evolved a variety of mating behaviors, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Commonly occurring sexual dimorphisms such as size and color differences are energetically costly attributes that signal competitive breeding situations.^[1] Many types of avian sexual selection have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signaling adaptations ensure that sexually selected ornaments such as plumage coloration and courtship behavior are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviors.^[2]

Bird species often demonstrate intersexual selection, perhaps because – due to their lightweight body structures – fights between males may be ineffective or impractical. Therefore, male birds commonly use the following methods to try to seduce the females:

• Colour: Some species have ornate, diverse, and often colourful feathers.
• Song: Male birdsong provides an important way of protecting territory (intrasexual selection).
• Nest construction: In some species, males build nests that females subject to rigorous inspection, choosing the male that makes the most attractive nest.
• Dance: Males dance in front of females. Cranes provide a well-known example.^[3]

As a propagandist, the cock behaves as though he knew that it was as advantageous to impress the males as the females of his species, and a sprightly bearing with fine feathers and triumphant song are quite as well adapted for war-propaganda as for courtship. —Ronald Fisher, 1930

In some bird species, both the male and the female contribute a great deal to offspring-care. In these cases, the male and female will be continuously assessing each other based on sexual characteristics. In the blue-footed booby, the females tend to choose males with brighter blue feet, because birds with brighter feet are younger, and thus have greater fertility and ability to provide paternal care.^[4] When researchers put make-up on the males' feet to make them look duller after the laying of the first eggs, their mates consequently laid smaller second eggs, which shows that female boobies continuously evaluate their mates' reproductive value.^[4] Males also vary their behaviour based on the females' foot colour. Males mated to females with brighter feet are more willing to incubate their eggs.^[5]