We studied visits to potential host nests by two avian brood parasites, the host generalist shiny cowbird, Molothrus bonariensis, and the host specialist screaming cowbird, Molothrus rufoaxillaris, in the periods preceding and overlapping the laying period of their hosts. Our goal was to examine the hypothesis that during prelaying visits, cowbird females form a dynamic memory library of laying opportunities, which they deploy to target suitable nests at a later predawn period. We recorded presence of radio-tagged females within a fixed area around nests of chalked-browed mockingbirds, Mimus saturninus (a common host of shiny cowbirds), and baywings, Agelaioides badius (the main host of screaming cowbirds), using proximity data loggers placed at nests during prelaying, laying and early incubation. Our data confirmed that females of both species visit potential host nests prior to laying and that parasitic events occurred before dawn, earlier in screaming than shiny cowbirds but with little chance of host nests having been discovered on the laying day. There were interesting species differences: visits were less frequent in shiny than screaming cowbirds and the former rarely returned after laying, while screaming cowbirds visited nests repeatedly after laying and occasionally showed repeat parasitism. The higher frequency of revisiting by screaming cowbirds to baywing nests is consistent with the uncommonly long and variable baywing prelaying period, and the incidence of repeat parasitism may reflect low availability of baywing nests and greater flexibility of its parental care potential thanks to it being a social breeder.
Avian brood parasites synchronize their laying with that of their hosts, as this reduces egg rejection and optimizes hatching time. They also avoid parasitizing nests repeatedly, thus preventing harm to their own previously laid eggs and competition among their offspring. Further, they lay at dawn, so that location of target nests must be known from previous days’ exploration. It has been argued that these adaptations must depend on memory for the location and status of host nests within their home range, a memory feat known as ‘bookkeeping’. We study nest prospecting in a host specialist and a host generalist parasitic cowbird, using a combination of proximity radio tracking and video recordings. Our results confirm the prospecting hypothesis, report previously unknown interspecies differences and illustrate how cognitive adaptations can be studied in the context of field behavioural ecology.
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We thank Fundación Elsa Shaw de Pearson for allowing us to conduct this research in Reserva El Destino and Victoria Weinsztok and Cynthia Ursino for help in the field. RCS was supported by a scholarship from the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). VDF and JCR are research fellows of CONICET. This work was supported by grants from Agencia Nacional de Promoción Científica y Tecnológica and the University of Buenos Aires. All work complied with Argentinean law.
The study was funded by research grants of the University of Buenos Aires (W808) and Agencia Nacional de Promoción Científica y Tecnológica (PICT-2011-0045).
Conflict of interest
The authors declare that they have no conflict of interest.
The study was conducted with the permission of the Provincial Organism for Sustainable Development (OPDS, Buenos Aires, Argentina) and complies with the current laws of Argentina.
Communicated by M. Soler
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Scardamaglia, R.C., Fiorini, V.D., Kacelnik, A. et al. Planning host exploitation through prospecting visits by parasitic cowbirds. Behav Ecol Sociobiol 71, 23 (2017) doi:10.1007/s00265-016-2250-8
- Brood parasitism
- Molothrus bonariensis
- Molothrus rufoaxillaris
- Nest searching
- Automated telemetry