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Acoustic discrimination by hosts favours vocal trickery in fledglings of the brood-parasitic screaming cowbird

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Abstract

Agonistic interactions between obligate avian brood parasites and their hosts can lead to the coevolution at any stage of the nesting cycle, yet adaptations and counter-adaptations at the fledgling stage are poorly known. Young of the host-specialist screaming cowbird (Molothrus rufoaxillaris) closely resemble those of its greyish baywing (Agelaioides badius) host in appearance and begging calls. This overall similarity has shown to be adaptive to escape host discrimination after leaving the nest, but the role of acoustic signals in host deception remained unclear. We examined whether baywing parents are able to distinguish between begging calls of mimetic and non-mimetic fledglings and whether screaming cowbirds can trick host parents by vocally resembling host young. We conducted a field playback experiment using fledgling calls of screaming cowbird (mimetic), shiny cowbird (M. bonariensis; non-mimetic) and baywing (conspecific control) in the absence of any visual stimuli. Baywings were significantly less responsive to non-mimetic shiny cowbird calls than to the other call types and more responsive to screaming cowbird calls than to conspecific calls. The results support the idea that baywings cue in on species-specific acoustic signals for fledgling recognition and that vocal similarity to host young in screaming cowbirds plays a role in host deception. The observed host preference for screaming cowbird over conspecific calls further suggests that vocal mimicry in brood parasites could be reinforced by the expression of acoustic features that either act as a supernormal stimulus or exploit host’s pre-existing sensory biases.

Significance statement

Brood-parasitic screaming cowbird juveniles closely resemble those of its primary host, the greyish baywing, in plumage appearance and vocalizations. Using a field playback experiment, we studied the ability of adult baywings to discriminate against foreign juveniles based on vocal cues, in the absence of any visual stimuli. Baywings responded differentially to own-species and non-mimetic calls, and even more, they were more attracted to screaming cowbird calls than to those of conspecific juveniles. The study supports the idea that baywings use acoustic signals to distinguish between their own and foreign fledglings, thus favouring the evolution of vocal mimicry in its specialist brood parasite. Moreover, the results suggest that screaming cowbird fledglings would be even more effective than host’s own young in attracting parental attention through their begging calls.

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Lama F, Ursino CA, Reboreda JC, De Mársico MC (2022) Acoustic discrimination by hosts favours vocal trickery in fledglings of the brood-parasitic screaming cowbird. Dryad, Dataset, https://doi.org/10.5061/dryad.wpzgmsbkh

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Acknowledgements

We thank Fundación Elsa Shaw de Pearson for allowing us to conduct this study at Reserva El Destino and Juan Manuel Rojas Ripari for their invaluable help with fieldwork. We are grateful to Maria Smith, Ros Gloag, Tomáš Grim and one anonymous reviewer for their insightful comments and suggestions on earlier versions of this manuscript.

Funding

CAU is supported by a Presidential Postdoctoral Research Fellowship of Princeton University. CAU, MCDM and JCR are fellows of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). This study was supported by grants to JCR and MCDM from Agencia Nacional de Promoción Científica y Tecnológica and University of Buenos Aires.

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Correspondence to Cynthia A. Ursino.

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Ethical approval

Experimental manipulations were conducted under a permit from the local authority (Organismo Provincial para el Desarrollo Sostenible, OPDS, Res. 202/12). All applicable national institutional guidelines for the use of animals were followed. We did not detect any negative effect of playback treatments on nesting success. Six of the 13 experimental nests were visited on the days following the playback experiment, and in all cases, we could corroborate nest success through direct observations of banded fledglings out of the nest. Baywings tolerated the experimental set-up well and they typically resumed their normal parental activity as soon as we placed the equipment and moved away from the nest. There was no case of brood abandonment or undernourishment following the experiment.

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All authors declare that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

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Communicated by M. Soler.

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Lama, F., Ursino, C.A., Reboreda, J.C. et al. Acoustic discrimination by hosts favours vocal trickery in fledglings of the brood-parasitic screaming cowbird. Behav Ecol Sociobiol 76, 71 (2022). https://doi.org/10.1007/s00265-022-03175-9

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  • DOI: https://doi.org/10.1007/s00265-022-03175-9

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