Vocal mimicry of host fledglings by a specialist brood parasite: a counter-defence against host discrimination?

DE MÁRSICO, M.C.; LAMA, F.; GANTCHOFF, M.; REBOREDA, J.C.

Pirenopolis

Congreso; XXIV International Bioacoustics Council Meeting; 2013

Obligate avian brood parasites lay their eggs in nests of individuals of other species (hosts), which rear the parasitic offspring at the expense of their own reproductive success. In response to fitness losses, hosts may evolve defences against parasitism that may, in turn, favour the evolution of counter-defences in parasite populations. The screaming cowbird (Molothrus rufoaxillaris) is a South American specialist brood parasite that uses primarily a single host, the baywing (Agelaioides badius). Screaming cowbird fledglings are indistinguishable from baywings in plumage colour, which has been proposed as a case of evolved mimicry in response to host discrimination against dissimilar young. We investigated whether host mimicry by screaming cowbirds extends to vocal cues. We quantifyed the degree of similarity in six begging call parameters (minimum and maximum frequency, bandwidth, peak frequency, number of syllables and call duration) among hand-reared baywing, screaming cowbird and ?non-mimetic? shiny cowbird (M. bonariensis) fledglings (n = 13, 13 and 20, respectively). A discriminant function analysis better distinguished between shiny cowbirds and the other two species than between baywing and screaming cowbirds on the basis of call duration and number of syllables, whereas peak frequency separated baywings from screaming cowbirds. Besides, we conducted a field experiment in which we broadcasted begging calls of baywing, screaming cowbird and shiny cowbird fledglings at 15 active baywing nests by the end of the nestling stage. Response of baywing parents was much stronger to ?mimetic? than ?non-mimetic? calls, as expressed by significantly shorter latencies, more time spent close to the speaker and more adults responding when broadcasting screaming cowbird and conspecific calls than shiny cowbird calls. Our results support the occurrence of vocal mimicry of host fledglings by screaming cowbirds and highlight its adaptive role in driving host attention towards parasitic young after leaving the nest.