Behaviour 2019
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Cooperative heterospecific eavesdropping on an anti-parasitic referential alarm call
Shelby Lawson1, Janice Enos1, Sharon Gill2, Mark Hauber1. 1University of Illinois at Urbana-Champaign, Champaign, IL, United States; 2Western Michigan University, Kalamazoo, MI, United States

Referential alarm calls can denote a variety of specific threats, all which require specific actions to evade (e.g. ground vs flying predator). Yellow warblers (Setophaga petechia) produce referential “seet” calls that signal to conspecifics nearby obligate brood parasitic brown-headed cowbirds (Molothrus ater), which lay their eggs in the nests of other species, including the warblers. Previous research found that during incubation stage, red-winged blackbirds (Agelaius phoeniceus), another cowbird host species, eavesdropped on warbler seet call playbacks and responded to them with similar aggression as to cowbird chatters and predator calls. In order to determine how blackbirds respond to seet calls based on brood parasitism risk, we presented the same playbacks during nestling stage, when risk of brood parasitism is low. We found blackbirds mediated their aggression towards both parasite-relevant heterospecific alarm signals (cowbird chatters and seet calls) based on their current risk of brood parasitism. Red-winged blackbirds appear to eavesdrop on yellow warbler referential seets and use them as a warning system for brood parasitism for their own frontline nest defenses.