Sexual selection in birds
Commonly occurring sexual dimorphisms such as size and color differences are energetically costly attributes that signal competitive breeding situations.
Conflicts between an individual fitness and signaling adaptations ensure that sexually selected ornaments such as plumage coloration and courtship behavior are "honest" traits.
[2] Bird species often demonstrate intersexual selection, perhaps because – due to their lightweight body structures – fights between males may be ineffective or impractical.
Different aspects and features of bird song such as structure, amplitude and frequency have evolved as a result of sexual selection.
In song sparrows, males with large repertoires had larger HVCs, better body condition and lower heterophil-to-lymphocyte ratios indicating better immune health.
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.
[9] Birds also use visual stimuli such as bright colors or large ornamentation for socio-sexual communication as well as species recognition.
[10] 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.
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.
[14][15] 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.
[16] 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.
Great bustards may eat toxic blister beetles of the genus Meloe to increase the sexual arousal of males.
[1] 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.
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.
The feathers coated with the preen oils look brighter and the degree to which the plumage was glossy was a way in which mates could determine the diet or overall health of the individual.
This study found that the flamingo applied these carotenoid rich secretions from the uropygial gland on its plumage and the resulting cosmetic coloration may influence mate choice.
Herons provide a great example of how cosmetic coloration is related to mate choice and sexual selection.
In the whistling heron, they develop a yellowish color on their neck, stomach, and tail due to the powder feathers.
In the spotless starling, it was found that individuals are able to identify the sex of conspecifics, or members of the same species, using the scent produced by the uropygial gland secretion.
[29] This evidence was supported by another study using a different species, the dark-eyed junco, showing that sex recognition by olfaction may be widespread in the taxa.
This species will preferentially orient to a specific tangerine-scented plumage odor during mechanistic courtship behavior that involves the smelling of the scented neck region.
[35] In species that frequently use aerial displays as a means of courtship behavior, the smaller, more agile males are selected for.
Antbird songs, which are sexually monomorphic ornaments, function as deterrents in competitive intrasexual interactions as well as in mate choice.
[41] Post-copulatory sexual selection is one of the main factors that drives the evolution of sperm morphology and ultimately its relative ability to fertilize an egg after copulation has occurred.
The best strategy for increasing the likelihood of extra pair fertilization is to time the copulation close to the onset of female oviposition.
[42] Some offensive adaptations include variable sperm morphology, testes size as well as strategies to evade mate guarding.
[43] In the superb fairywren, a socially monogamous species with a high frequency of extra pair copulations, the relative amount of extra-pair paternity was greater in individuals that had sperm with a shorter flagellum and a larger head.
[5] One mate guarding method is by following their fertilized female to prevent any extra-pair copulations which could decrease that particular male's chance of paternity.
