Excited to share new research with Mark Hauber's lab on the yellow warbler referential call system:
Lawson, S.L., J. K. Enos, N. C. Mendes, S.A. Gill, & M.E. Hauber. 2020. Heterospecific eavesdropping on an anti-parasitic referential alarm call. Communications Biology 3:143
For my MSc research, I investigated interactions between cowbirds and a common host, the yellow warbler. Yellow warblers are unusual in how they respond to cowbirds - giving a unique alarm call (called a "seet" call) and performing a type of nest-protection behavior which limits the access of cowbirds to the nest - compared with their generalized response to predators and non-threatening birds around their nests. I tested whether this specialized alarm call was communicated to other warblers that a cowbird was around the nest (such calls are termed "functionally referential"). This means that just hearing the alarm call, and not necessarily seeing or interacting with the cowbird, should be enough to elicit the same response as the cowbird itself.
The figure on the left shows how female warblers responded when they encounter a cowbird: they spent more time in their nests when confronting cowbirds than avian nest predators (data from Gill & Sealy 1996). The figure on the right shows how they respond to the seet call on its own: female warblers also sat in their nests, something they rarely did when their other alarm call, chips, were played (Gill & Sealy 2004). Females responded similarly to the cowbird and the seet call, which suggests that the seet call alone communicates the presence of cowbirds.
Alarm calls that function in this way are better known in primates and meerkats. In our recent review, we found strong evidence for functionally referential alarm calls in only six bird species, but more work needs to be done in other species which have promising signs of these unique calls (Gill & Bierema 2013).
Research on yellow warbler alarm calls is funded by a grant from the National Science Foundation (IOS-1953226).