Two novel vocalizations are used by veeries (Catharus fuscescens) during agonistic interactions.

Avian vocalizations are common examples of the complex signals used by animals to negotiate during agonistic interactions. In this study, we used two playback experiments to identify agonistic signals in a songbird species with several acoustically complex songs and calls, the veery. In the first experiment, we compared veery singing behavior in response to simulated territorial intrusions including playback of three variations of veery song: 1) song alone as a control, 2) songs with added whisper calls, and 3) songs with introductory notes removed. In the second experiment, we used multimodal stimuli including songs, whisper calls and songs with introductory notes removed, along with a robotic veery mount. Focal males readily responded to all of the playback stimuli, approached the speaker and/or robotic mount, and vocalized. Male veeries gave more whisper calls, and sang more songs without the introductory note in response to all types of playback. However, veeries responded similarly to all types of stimuli presented, and they failed to physically attack the robotic mount. These results indicate that rival veeries use two different types of novel vocalizations: whisper calls and songs lacking the introductory note as agonistic signals, but do not allow us to discern the specific functions of these two vocalizations.

Independent cultural evolution of two song traditions in the chestnut-sided warbler.

In oscine songbirds, song phenotypes arise via gene-culture coevolution, in which genetically transmitted learning predispositions and culturally transmitted song forms influence one another's evolution. To assess the outcome of this process in a population of chestnut-sided warblers (Dendroica pensylvanica), we recorded songs at intervals over a 19-year period. These recordings revealed the pattern of cultural evolution of songs in our study area, from which we inferred likely learning predispositions and mechanisms of cultural transmission. We found that the species' two song categories form two distinct cultural traditions, each with its own pattern of change over time. Unaccented-ending songs have undergone continual, rapid turnover of song and element types, consistent with a model of neutral cultural evolution. Accented-ending songs, in contrast, persisted virtually unchanged for the entire study period, with extraordinarily constant song form and only one appearance of a new song type. Our results indicate 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.