Reported last week in ScienceShot (a division of Science magazine) is news of a report due to be published in the journal The American Naturalist, that two different categories of birdsong evolve at radically different speeds. The paper, titled Independent Cultural Evolution of Two Song Traditions in the Chestnut-Sided Warbler was written by Bruce Byers, Kara Belinsky and R. Alexander Bentley, and describes the cultural transmission of songs sung by a population of chestnut‐sided warblers over a 19 year period. Specifically, the authors detail 2 types of songs sung by male warblers, one which is used to attract females, the other which is used to challenge another male. The main finding is that while the courtship songs persisted virtually unchanged in the populations discography during the 19 year study, the challenge songs displayed rapid and nearly continual changes. This distinction lead the authors to conclude that "in songbirds, multiple independent cultural traditions and probably multiple independent learning predispositions can evolve concurrently, especially when different signal classes have become specialized for different communicative functions."
What mechanisms underlie the different rates of song evolution? The authors discuss this at length, comparing the cultural pressures at work in the songbird community that might allow diversity to flourish in the fight songs, but constrain the mutation of courtship songs. Why would such constraint of mating song diversity be advantageous? "Stereotyped song forms that are few in number and shared widely among males provide unambiguous species identification (Marler 1960; Emlen 1972) and facilitate comparisons of individuals’ vocal performance (Zahavi 1980), especially if the stereotyped song also has features that make it difficult to perform (Ballentine et al. 2004; Podos and Nowicki 2004)."
The ScienceShot report extrapolates from this paper a comparison to human culture, comparing warbler fight songs to the yearly trends in baby names and pop music, and mating songs to the "strains of Jingle Bells [that] conjure holiday spirit year after year." And while the connection to holiday tunes is too tenuous for my taste, the question of what mechanisms underlie the concurrent, yet independent evolution of multiple cultural traditions within the warbler population could plausibly provide anthropologists with quantifiable data suitable for modeling such cultural evolution amongst human populations.