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Decision-making at the time of parasitism: cowbirds prefer to peck eggs with weaker shells

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Interspecific avian brood parasites, like cuckoos and cowbirds, lay their eggs in nests of other species, the hosts, which take over the entire parental care of the parasite's eggs and chicks. This breeding strategy requires decisions that may affect the parasite's reproductive success. During the breeding season, cowbirds search for host nests and revisit them to monitor its progress and parasitize at the time host laying begins. When visiting hosts nests, they repeatedly peck the nest contents trying to destroy one or more eggs. This behaviour favours parasite's offspring by reducing the competition for food with nestmates. We evaluated if the egg-pecking behaviour of female shiny (Molothrus bonariensis) and screaming (M. rufoaxillaris) cowbirds is affected by the strength and the size of the eggs they find in the nest. We presented to wild-caught females artificial clutches with two natural eggs that differ in size and shell strength. We found that female shiny and screaming cowbirds adjusted egg-pecking behaviour based on the strength but not on the size of the eggs. When differences in strength between eggs were high, both cowbird species pecked more frequently the egg with the weaker shell, increasing the probability of a successful puncture. Our results indicate that female cowbirds can discriminate eggs through the strength of the shell, and by choosing the weaker egg to peck, they increase the probability of puncturing.

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We thank the Elsa Shaw de Pearson Foundation for supporting our fieldwork at Reserva El Destino. We also thank two anonymous reviewers and the Associate Editor Kazuhiro Goto who provided their helpful comments on a previous version of the manuscript. This work was supported by Grants of the Agencia Nacional de Promoción Científica y Tecnológica and the University of Buenos Aires.


This work was supported by grants of the Agencia Nacional de Promoción Científica y Tecnológica and the University of Buenos Aires.

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Correspondence to Vanina D. Fiorini.

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This manuscript is not being considered elsewhere and all coauthors have agreed to this submission. We have no conflicts of interest to disclose.

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Experiment protocol has been established in compliance with the ethical standards, ensuring that all necessary precautions have been taken and the welfare of the birds has been respected. All work complied with the Argentinean Law for the Conservation of Wild Fauna (Ley Nacional de Fauna 22421/81) and was conducted with the authorization from the Organismo Provincial para el Desarrollo Sostenible, Argentina (Permit Number 202/12-O.P.D.S.).

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Cossa, N.A., Reboreda, J.C. & Fiorini, V.D. Decision-making at the time of parasitism: cowbirds prefer to peck eggs with weaker shells. Anim Cogn 25, 275–285 (2022).

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