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."