Vocal trickery allows brood-parasitic screaming cowbird juveniles to escape host discrimination after leaving the nest

LAMA, F.; URSINO, C.A.; REBOREDA, J.C.; DE MÁRSICO, M.C.

New York

Congreso; 15th Conference of the International Society of Behavioral Ecology; 2014

International Society of Behavioral Ecology

Avian brood parasitism imposes fitness costs to hosts, favoring the evolution of antiparasite defenses such as rejection of parasitic offspring. Parasites may evade host defenses by mimicking host eggs or young. The screaming cowbird (Molothrus rufoaxillaris) is a host-specialist parasite that uses primarily the baywing (Agelaioides badius). Begging calls of parasitic juveniles closely resemble baywings?, suggesting that screaming cowbirds might mimic acoustic signals of host juveniles to escape rejection. To test this idea we conducted two playback experiments at baywing nests during breeding seasons 2012-2013 and 2013-2014 (Dec-Feb) in eastern Argentina. In 2012-2013 we sequentially broadcast begging calls, matched for call rate and amplitude, of baywing, screaming cowbird and non-mimetic shiny cowbird (M. bonariensis) juveniles (n=15). In 2013-2014, we broadcast calls of these three species plus ?manipulated? screaming cowbird calls (i.e. natural calls hand-edited to resemble non-mimetic shiny cowbird in call duration and number of syllables; n=18). Baywings consistently responded faster to screaming cowbird than to shiny cowbird calls. Response duration, number of approaches to the loudspeaker and number of responding adults were higher for screaming cowbird than shiny cowbird calls in both seasons, and higher for screaming cowbird than for baywing calls in 2013-2014 but not in 2012-2013. Host responses to ?manipulated? screaming cowbird calls were intermediate between unmanipulated screaming cowbird and baywing calls, and higher than to shiny cowbird calls. Our results indicate that bawyings cue on begging call structure to discriminate foreign juveniles and screaming cowbirds escape discrimination by tuning into host preferences for species-specific acoustic traits.