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Avian species often display a variety of fascinating mating behaviors that have evolved as a result of sexual selection. Commonly occurring sexual dimorphisms such as size and color differences are energetically costly attributes that signal competitive breeding situations. Many types of sexual selection are often identified in avian species; 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.

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 (Melospiza melodia), 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 (Cettia diphone) songs from island populations have an acoustically simple structure when compared to mainland populations. This 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 of selection for auditory signals is in Purple-crowned fairy-wrens, Malurus coronatus coronatus. 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, these lower frequency calls are a form of honest signaling. This negative correlation between body size and call frequency is supported across multiple species within the taxa. In the case of the rock sparrow, Petronia petronia, 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 Taeniopygia guttata, also known as 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.

A classic example of how elaborate plumage ornamentation has evolved because of female choice is the male Pavo cristatus, commonly called the peacock. 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 a 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. Recently 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, however, 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. The peacocks train is also not a reliable indicator of the individuals 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 parrots, Psittasiformes, the more ornate males with brighter plumage are more preferred by the females. These males were 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, Callipepla californica, uses multiple male plumage characteristics when deciding on a mate and respond differently 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, Alectoris rufa, 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 (by means of paint) produced a larger quantity of eggs in less time than controls. The eggs produced were of similar quality in both cases showing that female red-legged partridges can adjust laying capacity based on the apparent carotenoid-based ornamentation of its mate.

Nest building is another way that birds can visually signal their fitness without the need for plumage ornamentation. European wren (Troglodytes troglodytes) males build multiple nests in their territory to display to females. A large number of complete but unoccupied nests gives the males a reproductive advantage. Most of these nests are never used for brooding and appear to be completely ornamental.

Olfactory Signaling
Avian olfactory signals are often overlooked as a form of communication. They possess developed olfactory apparatuses similar in function and structure to other vertebrates that are known to communicate chemically. Several species of birds have been found to discriminate sex using olfactory cues alone. In the spotless starling, Sturnus unicolor, 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. Uropygial secretion composition is known to differ among individuals. This evidence was supported by another study using a different species, the dark-eyed junco, Junco hyemalis, showing that sex recognition by olfaction may be widespread in the taxa. It was found that the juncos were able to identify the sex of conspecifics as well as body size.

Specific odors in crested auklets, Aethia cristatella, are directly related to courtship behavior. 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. This is one way odor transmission can occur for a sexually selective mate assessment.

Courtship Displays
Several species of birds are also know to combine visual and auditory stimuli in their elaborate courtship displays. The combination of song and dance to create a complex courtship display is favored by sexual selection, with females assessing the male’s ability to perform a well-choreographed display. Lyrebirds, Menura novaehollandiae, and manakins, Chiroxiphia linearis, both give elaborate displays involving vocal and non-vocal sound production as well as visual displays.

Male bowerbirds build elaborately decorated structures called bowers to attract mates. Bowers are hut or tower-like structures that are often decorated with sticks, fruits, flowers and stones. Amblyornis inornata, a species of bowerbird with the least plumage ornamentation (males and females are nearly identical), builds the most elaborately decorated bowers. This correlation shows that the female attention has changed from that of body ornamentation to that of bower complexity.

Acrobatic aerial displays are another behavior that is used to advertise fitness to potential mates. In species that frequently use aerial displays as a means of courtship behavior, the smaller, more agile males are selected for. In the case of the dunlin, Calidris alpina, display rate, as well as the proportion of time spent in aerial display is negatively correlated with male body size. This is one of the few courtship behaviors that lead to smaller males being selected for.

Male-Male Competition
Intrasexual competition in birds, which usually occurs between males in competition over dominance and territorial acquisition, leads to sexually selective pressures that can drive the evolution of specific traits.

Dominance Status
In brown-headed cowbirds (Molothrus ater), male displays were less intense when directed toward females than when directed to other males. The intense displays between males are most likely used to demonstrate condition and dominance status, and sometimes these displays escalate into physical fights in which the less dominant male is injured or killed. This is an example of when intrasexual competition requires more energy than the attraction of a mate.

Visual stimuli are also used in male-male competition. In rock sparrows, elaborate feather ornamentations are the best predictor of dominance in foraging groups. Using social network analysis to determine patterns of leader-follower interactions in the rock sparrow (Petronia petronia), individuals with the brightest yellow breast patches showed the most dominance in the foraging group and had the most followers compared to less elaborate individuals.

Several sexually selected ornaments that may be used during courtship can also be used as armaments. Antbird songs, which are sexually monomorphic ornaments, function as deterrents in competitive intrasexual interactions as well as in mate choice. In several species of antbirds (Hypocnemis), acoustic signals function in both intrasexual competition and mate choice. Removal experiments were performed to determine which function takes precedence. Both sexes use song as both ornament and as a form of competition, however, males demonstrate stronger signaling in both cases than females, giving more frequent signals and stronger responses.

Territorial Behavior
Before and after social pairing and mating occurs, bird song is mainly dedicated to territorial defense. This behavior is a sexually selected trait because ensures defense of the female who is rearing her offspring. There is also some evidence that vocal amplitude effects male-male competition in such species as the Great Tit, Parus major. Most courtship songs were performed at relatively low amplitudes, whereas territorial songs or “broadcast songs” were performed at high amplitudes. This suggests an environment where it is necessary to devote more energy to territory defense than to attracting a mate.

Performance level of territory defense song is important in the context of sexual selection. By manipulating the territory defense song of the banded wren (Thryophilus pleurostictus) to simulate three levels of song performance, banded wrens were much less likely to approach the high performance recording than the medium or low performance stimuli. Also, low performance stimuli were challenged without any further assessment.

Sperm Competition
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. Sperm competition occurs when a female is inseminated by multiple males during one breeding season resulting in differential fertilization success among males. In birds, the last male to inseminate the female usually fertilizes the highest proportion of eggs because by the time fertilization occurs, the oldest spermatozoa have been lost. This is known as last male sperm precedence. The best strategy for increasing the likelihood of extra pair fertilization is to time the copulation close to the onset of female oviposition.

Many male adaptations, both offensive and defensive, have been selected for due to this phenomena in a variety of avian species. Some offensive adaptations include variable sperm morphology, testes size as well as strategies to evade mate guarding. Morphological sperm traits such as flagellum, head and mid-piece length have been studied in several species of passerine birds to determine phenotypic correlations across species (Immler et al., 2012). There is large variation of sperm length in passerine birds. Total sperm length can vary from 50 to 300 μm. Several females of passerine species store sperm up to several weeks between insemination and fertilization. This has driven the evolution of sperm that is able to survive for longer periods of time. In these species, sperm with longer flagella, despite their ability to swim faster do not increase fertilization success because they require more energy and cause a shorter sperm lifespan. In the fairy-wren (Malurus cyaneus), 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. The males with longer flagella and smaller heads had higher within-pair paternity. Shorter sperm with large heads are more able to withstand long durations of storage whereas the opposite phenotype was better at outcompeting previously stored sperm.

Since fertilization chances for an individual male are proportional to the amount of sperm simultaneously transferred into a female, the size of the testes and resulting production of sperm increases in situations with high intrasexual competition. There is a negative correlation between testis size and variation in mate guarding behavior. In several species of the Australian Maluridae, as the competition level of sperm increases, testicular spermatogenic tissue also increases proportionately. This suggests that sperm competition selects for greater sperm production per unit volume of testicular tissue. The proportion of motile sperm in ejaculates was also greater in species that had the highest intrasexual competition.