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.
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
Birds also use visual stimuli such as bright colors or large ornamentation for socio-sexual communication as well as species recognition. These ornaments can be considered "honest" signs of fitness because they are often costly to produce and show that the individual is healthy enough to mate with the choosing female.
The peacock's plumage shows intersexual selection, where ornate males compete to be chosen by females. The result is a stunning feathered display, which is large and unwieldy enough to pose a significant survival disadvantage, demonstrating the handicap principle, and possibly provide a means of demonstrating body symmetry, such that peahens are "trying" to discover the health of the male or the fitness of his genes. Diseases, injuries, and genetic disorders may impair the body's symmetry and the tail.
In experiments where eyespots are removed from their tail feathers, also known as trains, there is a significant decline in mating success compared with a control group. This supports the hypothesis that the train elaboration evolved, at least in part, as a result of female choice. The most common explanation for this adaptation is that the females gain indirect benefits such as good genes for their offspring. Peafowl are lekking species and males provide no care to their offspring, therefore females do not gain any direct benefits from mating with a more elaborate male. However, conflicting evidence has been found that removal of a large number of eyespots (≤20) from a male's train does reduce his mating success, although this is outside the natural variation of eyespot loss. This shows that peafowl preference is more complex than originally thought. Takahashi et al., 2008, found no evidence that peahens expressed any preference for males with more elaborate trains. This shows that trains are not the universal target for female choice due to their small variance among males across populations. A peacock's train is also not a reliable indicator of the individual's condition. Although the train may be necessary for mating success, the females seem to be more affected by the males' behavioral characteristics during courtship.
In great bustards, age, weight, and display effort are all significant and independent predictors of male mating success, where whiskers and neck plumage are reliable indicators of male age and weight. A male's sexual display consists of a series of extravagant body postures and movements that end, when an interested female approaches, with a reiterative and almost obstinate exhibition of the cloaca, which is fully surrounded by pure white feathers that allow an easy detection of possible parasites or their remains. Exposure of the cloaca is widespread in pre-copulatory displays of birds and has been related with high probability of transmission of sexual diseases. Male great bustards may use the highly toxic cantharidin taken from blister beetles of the genus Meloe to clean their intestine and cloaca plumage from parasites. Cantharidin can kill a great bustard if many beetles are ingested. Self-medication could have evolved in great bustards as a sexual selection mechanism capable of transmitting to females a signal of good resistance to a poisonous compound, in a similar way as other costly secondary sexual traits are exhibited by males of many species. Great bustards may eat toxic blister beetles of the genus Meloe to increase the sexual arousal of males.
In parrots, ornate males with brighter plumage are preferred by the females. These males are typically immunologically superior with higher leukocyte counts. This evidence supports the idea that bright plumage is an "honest" signal involved in mating. The female California quail uses multiple male plumage characteristics when deciding on a mate and responds in different ways to a variety of artificially manipulated traits. Various visual signals act in combination to attract a mate and female choice will shift toward several particularly exaggerated traits. In the red-legged partridge, male carotenoid ornamentation is positively correlated with relative reproductive investment of the female. This species has variable egg laying capacity and females who mated with color-enhanced males produced a larger quantity of eggs in less time than controls. The eggs produced were of similar quality in both cases showing that the females can adjust laying capacity based on the apparent carotenoid-based ornamentation of its mate.