Sharon A. Gill, Spencer G. Sealy, 2004
Functional reference in an alarm signal given during nest defence: seet
calls of yellow warblers denote brood-parasitic brown-headed cowbirds
Behavioral Ecology and Sociobiology,56(1): 71-80.
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Abstract Field observations and model-presentation experiments have shown
that yellow warblers (Dendroica petechia) produce seet calls preferentially
in response to brood-parasitic brown-headed cowbirds (Molothrus ater). In
this study, we investigated whether seet calls are functionally referential
alarm calls denoting cowbirds by determining whether female warblers
responded appropriately to seet calls in the absence of a cowbird, whether
alarm calling by warblers varied with response urgency, and how warblers in
a population allopatric with cowbirds responded to cowbird and avian
predator models and seet playbacks. As a control, we presented chip calls,
which are elicited by nest predators as well as by non-threatening
intruders, but are not strongly associated with cowbirds. Yellow warblers
responded differently to playbacks of seet than chip calls. To seet
playbacks, almost 60% of females gave seet calls and rushed to sit in their
nests, responses typically elicited by cowbirds, whereas these responses
were given infrequently in response to chip calls. Yellow warblers seet
called equally in situations that simulated low, medium and high risk of
parasitism, which suggests that these calls did not vary with response
urgency. In a population allopatric with cowbirds, seet calls were rarely
produced in response to cowbird or avian nest predator models and never to
seet playbacks. These results suggest that seet calls are functionally
referential signals denoting cowbirds and that cowbird parasitism was a
strong selective pressure in the evolution of functional referentiality in
the seet call of yellow warblers.
