Territorial African wood-doves respond differently to intruders with higher and lower frequency of songs

This study aimed to examine the potential influence of long-distance acoustic signal frequency produced by the blue-headed wood-doves, Turtur brehmeri , on responses to a simulated territorial intrusion by a stranger male. The blue-spotted wood-dove is an African equatorial forest inhabitant that defends long-term, stable territories and exhibits different responses to known neighbours and unknown strangers. Males repeat their single song with a highly repetitive structure, including frequency parameters. Although the limited sample size reduced our power to detect subtle associations, our results do not provide evidence for a within-species relationship between body size and song frequency. Song frequency may nevertheless influence territorial interactions through mechanisms other than signalling body size. Hypothesizing that these doves tune their response to unknown rivals based on the perception of their song frequency, we predicted that males differentiate the strength of their response to intruders characterised by relatively low or high song frequency and should respond stronger or more cautious to songs of intruder which are lower than their own song frequency, in comparison to the songs which have higher frequency. A total of 25 males were tested twice, in random order, with either a low- or a high-frequency song of the unknown intruder. Birds differed significantly in their responses and avoided approaching the playback in the low-frequency treatment, which we interpreted as more cautious response, as staying at a greater distance from the speaker decreases the chance of a physical fight. We also observed a single male attempting to lower the frequency of his song in response to low-frequency playback, which, however, resulted in distortion of the phrase structure. The findings of this study enhance our understanding of how birds, especially doves, perceive differences in song frequency. They clearly indicate that birds take this characteristic into account even when it is not strongly linked to simple body features associated with size. Repeatability of song structure and rare attempts to change the frequency suggest that the change in frequency is constrained by a production mechanism or by vocal-tract characteristics.