Egg pecking and puncturing behaviors in shiny and screaming cowbirds: effects of eggshell strength and degree of clutch completion

  • Natalia A. Cossa
  • Diego T. Tuero
  • Juan C. Reboreda
  • Vanina D. Fiorini
Original Article


Shiny cowbirds (Molothrus bonariensis) are generalist brood parasites that use hosts varying in body and egg size. On the contrary, screaming cowbirds (M. rufoaxillaris), which are larger than shiny cowbirds, are host specialist that use mainly one host of similar body and egg size. Both parasites peck and puncture eggs when visiting nests. Through puncturing eggs, cowbirds can reduce the competition for food their chicks face (reduction of competition hypothesis), but the same behavior could also be a mechanism to enforce host to renest when nests are found late in the nesting cycle (farming hypothesis). Eggshell strength increases the difficulty to puncture eggs and therefore may modulate egg-pecking behavior. To test these hypotheses, we studied the effect of the degree of clutch completion and egg size on egg-puncturing behavior. Moreover, we evaluated if morphological differences between cowbird species and eggshell strength affected egg-pecking behavior. We presented captive females a nest with complete (four eggs) or incomplete (one egg) clutches of house wren (small egg size, low eggshell strength), chalk-browed mockingbird (large egg size, intermediate eggshell strength), or shiny cowbird (medium egg size, high eggshell strength). The proportion of nests with punctured eggs was similar for complete and incomplete clutches. Cowbirds punctured more eggs in complete than in incomplete clutches, but in complete clutches, they did not destroy the entire clutch. There were no differences in the egg-pecking behavior between cowbird species, which pecked more frequently the eggs with the strongest eggshell. Our findings are consistent with the reduction of competition hypothesis.

Significance statement

Brood parasitic birds do not build nests and raise their chicks. Instead, they lay eggs in nests of other species (hosts), which carry out all parental care. Some brood parasites, like the cowbirds, peck and puncture eggs when they visit host nests. This behavior may help to reduce the competition for food that their chicks face in the nest (reduction of competition hypothesis) or may enforce hosts to renest (farming hypothesis). We experimentally studied egg-pecking and egg-puncturing behaviors in the host generalist shiny cowbird and the host specialist screaming cowbird. We found that the degree of clutch completion and egg size modulate egg-puncturing behavior and eggshell strength modulates egg-pecking behavior. Our results indicate that by puncturing eggs, cowbirds reduce nest competition.


Brood parasitism Farming hypothesis Molothrus bonariensis Molothrus rufoaxillaris Reduction of competition hypothesis 



We thank “Elsa Shaw de Pearson Foundation” for supporting our fieldwork at Reserva El Destino. We are grateful to Brian Peer for thoughtful comments that contributed to the improvement of the manuscript. We also thank three anonymous referees and Manuel Soler 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. DTT, JCR, and VDF are research fellows of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. 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 de Desarrollo Sostenible, Argentina (Permit number 202/12-O.P.D.S.).

Supplementary material

265_2017_2289_MOESM1_ESM.mp4 (35.2 mb)
ESM 1 Video of shiny cowbird female pecking one shiny cowbird egg (MP4 36,078 kb).


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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • Natalia A. Cossa
    • 1
  • Diego T. Tuero
    • 1
  • Juan C. Reboreda
    • 1
  • Vanina D. Fiorini
    • 1
  1. 1.Departamento de Ecología, Genética y Evolución & IEGEBA-CONICET, Facultad de Ciencias Exactas y NaturalesUniversidad de Buenos AiresBuenos AiresArgentina

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