Summary: Kunc et al. (2006)

It is thought that overlapping of song in birds is a signal of immediate aggression. However, some evidence suggests that song overlapping may provide information to females about a male's quality, allowing them to make extrapair mating decisions based on male vocal performance (eavesdropping). Kunc et al. (2006) investigated how male behaviour (overlapping versus non-overlapping) influenced subsequent pairing status and how males respond to playbacks of vocalizations, using specific structural song components. They predicted that more successful males would overlap songs more if song is a signal of male quality. They found that males that mated during the breeding season overlapped their songs more to the playback than males that remained unpaired, suggesting that overlapping is a signal of male status. They also found that males adjusted the use of specific song components and decreased song rate, suggesting that flexibility of structural song components is more important than increasing song output. Since song overlapping may be a signal of aggression, Kunc et al. (2006) suggested that more aggressive males may have greater pairing success.

Summary: Sober & Brainard (2009)

Humans use vocal imitation from auditory feedback when learning to talk. Similarly, during the process of imprinting, young birds learn to imitate the songs of adults (tutors) through a reliance on auditory feedback. Initially discordant vocalizations are crystallized into mature song similar to their demonstrator's. During adulthood, humans continue to rely on auditory feedback to correct vocal errors. However, it is unclear whether parallel processes drive the stability of adult vocal behaviour. Sober & Brainard (2009) tested the hypothesis that adult Bengalese finches maintain vocal output by disturbing the pitch (fundamental frequency) of auditory feedback (using custom-designed headphones) and monitoring any resulting vocalization modifications. They predicted that shifts in auditory feedback pitch would cause birds to change their own song pitch in the direction opposite to the imposed feedback. They found that birds use auditory feedback and maintain their song vocalizations through a continual process of error correction. The birds adjusted their song pitch, thereby compensating for the auditory error imposed. Sober & Brainard's (2009) result show that error correction, even in adulthood, is a general principle of learned vocal behaviour.

Summary: Cardoso & Atwell 2011

Some oscines (songbirds) shift to singing at higher frequencies in urban areas (high noise-polluted areas), which has been interpreted as an adjustment to reduce acoustic masking by low-frequency man-made noise. Furthermore, they may also sing louder (i.e. higher amplitude) when exposed to noise and frequency shifting is thought to be less efficient than merely singing louder. Increasing vocalization amplitude in response to background noise is known as the Lombard effect. It has been suggested that high frequency singing might be a physiological consequence of louder singing, rather than just a a functional adjustment to noise. Cardoso & Atwell 2011 tested whether louder oscine songs are sung at higher frequency (central tenet of the hypothesis) using the dark-eyed junco, Junco hyemalis thurberi. The frequency bandwidth of songs and syllables increased with amplitude, involving lower minimum frequency in louder songs and syllables. Therefore, louder singing does not explain the higher minimum frequency of urban dark-eyed juncos. Amplitude and peak frequency were weakly positively related across but not within songs, suggesting that increased frequency is not an obligatory outcome of singing louder. Instead, birds may adjust both amplitude and frequency in response to changing noise or motivation across songs. Their results suggest that adjustments in song frequency and amplitude are largely independent and thus can be complementary rather than alternative vocal adjustments to noise. They discuss oscine vocal physiology and details of the behaviour of urban birds, both of which we argue are consistent with the increased frequency of urban birdsong generally being a functional adjustment to noise, rather than a consequence of singing louder.

Summary: Choi & Bowles (2007)

As in paper:
"Altruism—benefiting fellow group members at a cost to oneself—and parochialism—hostility toward individuals not of one’s own ethnic, racial, or other group—are common human behaviors. The intersection of the two—which we term “parochial altruism”—is puzzling from an evolutionary perspective because altruistic or parochial behavior reduces one’s payoffs by comparison to what one would gain by eschewing these behaviors. But parochial altruism could have evolved if parochialism promoted intergroup hostilities and the combination of altruism and parochialism contributed to success in these conflicts. Our game-theoretic analysis and agent-based simulations show that under conditions likely to have been experienced by late Pleistocene and early Holocene humans, neither parochialism nor altruism would have been viable singly, but by promoting group conflict, they could have evolved jointly."

Summary: Wittkopp et al. (2009)

Intra- and interspecific phenotypic variation may be influenced by genetic mechanisms, but the allelic relationship underlying intraspecific polymorphism and interspecific divergence is unknown. Wittkopp et al. (2009) examined the genetic basis of variation in pigmentation within and between two closely related Drosophila species (D. americana and D. novamexicana) to investigate the relationship between intraspecific polymorphism and interspecific divergence. They found that changes linked to both the tan and ebony genes in closely related Drosophila species contributes to divergence in pigmentation. Furthermore, they found that the alleles linked to these two genes in one species contributed to the variation in the other species, suggesting the similar phenotypes in a population may have different underlying genotypes. The genetic variation present in the common ancestor of these two species likely gave rise to both intraspecific polymorphism and interspecific divergence, as the alleles appear to predate speciation.

Summary: Griggio et al. (2010)

Preening behaviour is important for plumage maintenance in birds but has received relatively little attention with respect to the role it plays in influencing plumage colouration. Furthermore, elaborate or colourful plumage is important in female mate choice assessment in birds, but few studies have investigated the costs associated with maintaining this trait in good condition (through preening behaviour). Griggio et al. (2010) investigated whether preening behaviour of captive-bred, wild-type budgerigars, Melopsittacus undulatus, changes plumage reflectance and whether females exercise a choice (in two-choice tests) for males that have higher plumage reflectance (i.e. whether female prefer males that were allowed to preen over those prevented from preening or those who could preen, but had plumage what was UV-blocked by UV-absorbing chemicals). Griggio et al. (2010) found that the plumage of unpreened birds showed lower UV reflectance and females showed a preference for preened males. However, when females were given a choice between unpreened and UV-blocked males, they did not show a preference. Their results show that preening mediates plumage colouration (in the UV range) and can convey honest information about a bird's present condition. Furthermore, females are able to exercise a choice based on the level of UV reflectance.

Summary: Yoon et al. (2008)

Humans have a remarkable ability to learn socially, and teaching is a specialised form of cooperative information transmission. It is dependent on the presence of benevolent communication and on the readiness of students to learn. Yoon et al. (2008) hypothesized that human infants should be biased to perceive and member referents of communication, if they are sensitive to those signals typically indicating communication. They tested this hypothesis using 9-month-old human infants, asking whether the infants could discriminate between communicative and noncommunicative social contexts and whether they retain qualitatively different information about novel objects in these contexts. Yoon et al.'s (2008) results show that 9-month-old infants remember the identity of novel objects, but not the location, in a communicative context, while they remember the location of novel objects, but not the identity, in a noncommunicative context. Yoon et al. (2008) propose that infants are sensitive to, and interpret social cues that indicate specific intentions and these represent important mechanisms of social learning, by which others can help determine what information observers retain in memory.