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. 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. 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. 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. 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. 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.
Acoustic signaling
One of the most prominent forms of avian communication is by means of acoustic signals. These signals are widespread in avian species and are often used to attract mates. Different aspects and features of bird song such as structure, amplitude and frequency have evolved as a result of sexual selection. Large song repertoires are preferred by females of many avian species. One hypothesis for this is that song repertoire is positively correlated with the size of the brain's song control nucleus (HVC). A large HVC would indicate developmental success. In song sparrows, males with large repertoires had larger HVCs, better body condition and lower heterophil-to-lymphocyte ratios indicating better immune health. This supports the idea that song sparrows with large song repertoires have better lifetime fitness and that song repertoires are honest indicators of the males "quality." Possible explanations for this adaptation include direct benefits to the female, such as superior parental care or territory defense, and indirect benefits, such as good genes for their offspring. Japanese bush warbler songs from island populations have an acoustically simple structure when compared to mainland populations. Song complexity is correlated with higher levels of sexual selection in mainland populations, showing that a more complex song structure is advantageous in an environment with high levels of sexual selection. Another example is in purple-crowned fairywrens; larger males of this species sing advertising songs at a lower frequency than smaller rival males. Since body size is a characteristic of good health, lower frequency calls are a form of honest signaling. Negative correlation between body size and call frequency is supported across multiple species within the taxa. In the rock sparrow, song frequency is positively associated with reproductive success. Slower song rate is associated with age and is preferred by females. Reproductive status of the individual is communicated through higher maximum frequency. There was also positive correlation between age and extra-pair copulation frequency. Bird calls are also known to continue after pair formation in several socially monogamous bird species. In one experimental population of zebra finches, there was increased singing activity by the male after breeding. This increase is positively correlated with the partner's reproductive investment. The female finches were bred in cages with two subsequent males that differed with varying amounts of song output. Females produced larger eggs with more orange yolks when paired with a male with a high song output. This suggests that the relative amount of song production in paired zebra finch males might function to stimulate the partner rather than to attract extra-pair females.
Visual signaling
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