Red-winged Blackbird eavesdropping nestling

Referential alarm calls that denote specific types of dangers are common across diverse taxa. Different calls can indicate a variety of threats, which often require specific actions to evade. Thus, to benefit from the call, listeners of referential alarm calls must be able to decode the threat being signaled, and respond in an appropriate manner. Yellow warblers (<i>Setophaga petechia</i>) produce referential “seet” calls that signal to conspecifics of nearby obligate brood parasitic brown-headed cowbirds (<i>Molothrus ater</i>), which lay their eggs in the nests of other species, including yellow warblers. Previous playback experiments found that red-winged blackbirds (<i>Agelaius phoeniceus</i>), a heterospecific co-occurring species also parasitized by brown-headed cowbirds, eavesdrop on and respond to yellow warbler seet calls during the incubation stage of nesting with similar aggression as to both cowbird chatters and predator calls. To determine if red-winged blackbird responses to seet calls vary with brood parasitism risk, we presented the same playbacks at nests during the nestling stage, when risk of brood parasitism is low. We found that blackbirds mediated their aggression towards parasite-relevant heterospecific alarm signals (cowbird chatters and seet calls) in parallel with their current risk of brood parasitism (i.e., lower to during the nestling than the egg stages). These results further support that blackbirds respond to yellow warbler antiparasitic referential calls as a frontline defense against brood parasitism at their own nests.