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For larger hosts, cowbirds lay eggs in nests with smaller eggs

Cara J

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The brown-headed cowbird (Molothrus ater) may not build nests or raise young, but it does seem to take steps to help its offspring’s survival. When it lays eggs in the nests of larger species, new research from Illinois suggests, it chooses nests with eggs that are smaller than usual for the host species, which may help the young cowbirds compete for resources with the foster mother’s hatchlings. “The results suggest cowbirds are not indiscriminate layers,” said Loren Merrill, first author on the study published in Oecologia. “They appear to exhibit a high degree of choosiness.” From 2011 to 2015, Merrill, a postdoctoral researcher at the Illinois Natural History Survey, and his colleagues searched 16 shrubland sites throughout the state. They found more than 3,000 nests where the parasitic cowbird could have left its eggs after the foster female laid its own eggs. The biologists measured the size of cowbird and host eggs in over 500 nests they located. Among foster species bigger than the cowbird, the researchers observed that nests containing smaller-than-average host eggs were more likely to be parasitized by cowbirds. Female cowbirds could be selecting nests where their eggs and chicks face less competition, Merrill said. If a [...]


Read more: http://wildlife.org/for-larger-hosts-cowbirds-lay-eggs-in-nests-with-smaller-eggs/

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