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Parasitic cowbirds and cuckoos generally reduce the clutch size of the hosts they parasitize by removing or destroying some of their eggs. Shiny cowbirds (Molothrus bonariensis) puncture their host's eggs both when parasitizing the nests and also when they do not parasitize them. We propose that, by puncturing the host's eggs, shiny cowbirds gain an informational benefit. They assess the degree of development of the host's embryos and so avoid laying in nests that would not provide enough incubation time for the parasitic eggs to hatch. Two predictions follow: (i) punctures should occur in advance or immediately before parasitic events, and (ii) the occurrence of parasitism should depend on the degree of development of the host's embryos when punctures occurred, i.e. on the stage of incubation. Both predictions are supported by our data of shiny cowbirds parasitizing yellow–winged blackbirds (Agelaius thilius). Egg punctures are not used to reset the host's nesting attempt when shiny cowbirds do not parasitize the nests. We discuss the potential mechanisms implicated in egg development assessment and propose a critical experiment to test this hypothesis.

Egg puncture allows shiny cowbirds to assess host egg development and suitability for parasitism