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Shiny cowbirds share foster mothers but not true mothers in multiply parasitized mockingbird nests

Abstract

Obligate brood parasitic birds, such as cowbirds, evade parental care duties by laying their eggs in the nests of other species. Cowbirds are assumed to avoid laying repeatedly in the same nest so as to prevent intrabrood competition between their offspring. However, because searching for host nests requires time and energy, laying more than one egg per nest might be favoured where hosts are large and can readily rear multiple parasites per brood. Such ‘repeat parasitism’ by females would have important consequences for parasite evolution because young parasites would then incur indirect fitness costs from behaving selfishly. We investigated shiny cowbird (Molothrus bonariensis) parasitism of a large host, the chalk-browed mockingbird (Mimus saturninus), in a population where over 70 % of the parasitized mockingbird nests receive multiple cowbird eggs. We assessed egg maternity directly, using cameras at nests to film the laying of individually-marked females. We also supplemented video data with evidence from egg morphology, after confirming that each female lays eggs of a consistent appearance. From 133 eggs laid, we found that less than 5 % were followed by the same female visiting the nest to lay again or to puncture eggs. Multiple eggs in mockingbird nests were instead the result of different females, with up to eight individuals parasitizing a single brood. Thus, while cowbird chicks regularly share mockingbird nests with conspecifics, these are unlikely to be their maternal siblings. Our results are consistent with shiny cowbird females following a one-egg-per-nest rule, even where hosts can rear multiple parasitic young.

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Acknowledgments

We thank our anonymous reviewers for their helpful comments on this manuscript. We thank Juan Shaw and the Elsa Shaw de Pearson Foundation for supporting our fieldwork at Reserva El Destino. We also thank Romina Scardamaglia, María Cecilia De Mársico and Cynthia Ursino, for helping to trap birds in the field, and the five volunteers that performed the scoring of eggs. We are grateful to Agencia Nacional de Promoción Científica y Tecnológica (Proyecto PICT Raíces 2011-0045) and the Cogito Foundation for funding. RG was supported by the Cogito Foundation. JCR and VDF are Research Fellows of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).

Ethical standards

This research was conducted in accordance with relevant Argentinean regulations (Law of Conservation of Wild Fauna), under the permit issued to JCR, University of Buenos Aires.

Author information

Correspondence to Ros Gloag.

Additional information

Communicated by M. Soler

Electronic supplementary material

Below is the link to the electronic supplementary material.

An example of the recording of a multiply parasitized mockingbird nest, showing how nest cams and individually marked females were used to determine the maternity of cowbird eggs in the same nest (MPG 62312 kb)

Online Resource 1

An example of the recording of a multiply parasitized mockingbird nest, showing how nest cams and individually marked females were used to determine the maternity of cowbird eggs in the same nest (MPG 62312 kb)

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Gloag, R., Fiorini, V.D., Reboreda, J.C. et al. Shiny cowbirds share foster mothers but not true mothers in multiply parasitized mockingbird nests. Behav Ecol Sociobiol 68, 681–689 (2014). https://doi.org/10.1007/s00265-014-1682-2

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Keywords

  • Brood parasitism
  • Egg morphology
  • Mimus saturninus
  • Molothrus bonariensis
  • Multiple parasitism
  • Parental investment
  • Repeat parasitism