Male territorial aggression does not drive conformity to local vocal culture in a passerine bird

Abstract

In many songbird species, young individuals learn songs from neighbors and then settle nearby, thus creating neighborhoods of conformity to local vocal culture. In some species, individuals appear to postpone song learning until after dispersal, possibly to facilitate conformity to local dialects. Despite decades of study, we still lack a consensus regarding the selective pressures driving this delayed song learning. Two common hypothetical benefits to conformity, and thus delayed song learning, are rooted in territorial interactions; individuals preferentially produce local song either to avoid detection as new arrivals (deceptive mimicry) or to be more effectively recognized as conspecific territory holders. The dickcissel (Spiza americana) is an ideal species in which to study these hypotheses. Males of this species appear to delay song learning until they arrive at their first adult territory, each individual sings a single song type, and conformity to the local song culture is high. Using playback, we contradicted both of the territorial hypotheses described above; male dickcissels did not respond differentially to local vs foreign song playback treatment. We are confident in this lack of difference because dickcissels clearly responded less strongly to a third treatment, neighbor song, than to the other two treatments, demonstrating sufficient power in our experimental design (and providing the first evidence of the dear‐enemy effect in dickcissels). Our results raise the question of why dickcissels respond equally aggressively to both local and foreign songs when other bird species often show reduced aggression toward foreign song. If reduced aggression to foreign song is not ubiquitous in species that achieve conformity through delayed learning, then selection from aggressive territorial interaction seems unlikely to be a general explanation for such delayed learning. Reduced aggression in response to foreign songs in other species may be due to reduced exposure to the stimulus of foreign song or to different cost‐benefit trade‐offs when responding to songs that deviate from the local average.

Subspecies status and methods explain strength of response to local versus foreign song by oscine birds in meta-analysis

“…The differential response to local versus foreign song was considerably weaker when measured by calling or singing rate than by variables that capture body movement such as approach distance or flight rate. We expect that this is not because song is consistently a poor indicator of aggression, but rather that in some species elevated singing rate is not a primary response to detection of a territorial intrusion, and that in these species, direct approach, possibly with the intent to chase the rival from the territory, is the primary response (Parra et al 2017). Thus we suggest that future playback studies, especially those working in unfamiliar systems, use measurements of approach to the speaker as the index of response.…”

“…We had a strong a priori expectation that, at least in some systems, not all categories of response convey the same information. For instance, in some species individuals reduce song rate in response to playback and instead increase flights and approach the speaker (Parra et al 2017). Thus it may be that song rate is a less reliable measure of aggressive response to playback than approach to speaker.…”

“…Our indices of potential exposure to foreign song are also imperfect, and tests of the exposure hypothesis would benefit from detailed natural history knowledge about each studied population (Parra et al 2017). Ideally, we would know the spatial scale over which vocal culture changes, the likelihood of individual movements at those scales, and the correspondence between those movements and seasonal variation in singing.…”

“…For instance we assume that movement outside of the breeding season corresponds to exposure to foreign song. This is certainly true in some species (DeWolfe and Baptista 1995;Parra et al 2017), but in some others singing may be too limited outside of the breeding season to provide migrants with meaningful exposure to diverse conspecific signals. As with the acoustic characterization of the communities in which all these experiments were conducted, obtaining the extensive knowledge needed across a large number of populations is a major undertaking that we must leave to future researchers.…”