Agonistic singing behaviour in Adelaide's warbler (Setophaga adelaidae)

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Mower, Peter C.
University of Lethbridge. Faculty of Arts and Science
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Lethbridge, Alta. : University of Lethbridge, Dept. of Psychology
Animals use agonistic signals to mitigate the cost of conflict over resources. Bird song has long been thought to function as an agonistic signal in songbirds. Many songbird species vary aspects of their singing behaviour in and around agonistic encounters. For example, low amplitude “soft song” often precedes attack, and song rate can increase with aggressive motivation. In agonistic encounters, singing behaviour may be used to fill a number of agonistic signalling functions like communicating aggressive intent, social dominance or submission, and victory. Most of the studies on singing behaviour as a signal in agonistic contexts, however, rely on song playback to simulate an agonistic context. Remarkably few studies link singing behaviour to agonism in unmanipulated, natural systems. Adelaide’s warblers (Setophaga adelaidae) are an ideal study species in which to study natural agonistic signalling because neighbours frequently engage in conspicuous territorial skirmishes. I analyzed focal recordings of 23 individually marked males and characterized three singing behaviours around the time of natural agonistic encounters (n = 4,531 songs, 254 encounters): song type switch rate, within-repertoire song type frequency, and song rate. As predicted, results from Bayesian mixed-effects models indicate that all three singing behaviours vary with time to the nearest agonistic encounter. Song type switch rate dropped substantially before and after agonistic encounters, reaching a low point several minutes after the encounter, which may indicate its use as a post-conflict agonistic signal like a victory signal. Song type frequency increased slightly before an encounter, and returned to baseline well after the encounter was over. Though the effect is small, it could indicate that frequently-used song types are used as a signal of aggressive motivation. Finally, song rate decreased moderately immediately before encounters and returned to normal levels shortly after the encounter. This drop in song rate may be a signal of aggressive intent. In all three behaviours, the magnitude of this behavioural variation exceeds that produced by randomized data. However, effect sizes varied from small (in the case of song type frequency) to moderate (song rate) to large (song type switch rate) and alternative hypotheses may better explain some of these patterns, such as interactions between the singing behaviour and other agonistic behaviour.
agonistic singing behaviour , Adelaide's warbler , natural agonistic encounters , songbirds , song type switch rate , within-repertoire song type frequency , song rate