To trill or not to trill? Territorial response to a heterospecific vocal trait in male collared doves, Streptopelia decaocto
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Responding of individuals outside the conspecific range has been largely explained by biases in sensory or cognitive systems toward particular traits or trait values. More recently, it has been shown that such responses might occur if individuals still respond to signal traits that have been lost over time. However, empirical evidence remains scarce. We report a case supporting the latter mechanism. Phylogenetic analysis suggests that the collared dove, Streptopelia decaocto, had lost the trilled vocalization present in most congeneric taxa. We tested whether males retained the ability to respond to these trills in the context of territory defense. We synthesized trilled songs by inserting trills from the sister species S. roseogrisea into S. decaocto songs. We show that trilled songs yielded higher responses than did natural conspecific songs, and that the intensity of the response depended on the number of trilled elements. We also show that trilled songs elicited as strong reactions as frequency modulated coos, which are stronger releasers of territorial response than are nonmodulated coos, but are not produced by every males. Additional tests suggest that the frequency pattern is the most important feature of the trill used by males. However, it is still unknown whether reactions to trilled and frequency modulated coos have the same perceptual basis. To our knowledge, this study is the first report of a strongly deviating signal that is still effective in vocal intrasexual communication in birds.