Cowbirds eye egg size for nest selection

By Julia John

A female brown-headed cowbird perches on a branch in a shrubland region in Illinois. ©Loren Merrill

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 cowbird’s eggs aren’t much smaller than the host’s, the cowbird eggs have a higher chance of successful incubation, he said, and its chicks are more likely to obtain enough food and space for survival.

“No one, to our knowledge, had documented this level of apparent discrimination for wild cowbirds selecting among hosts to parasitize,” Merrill said.

Cowbirds deposit eggs in the nests of over 200 other species, some of which are bigger ones such as redwing blackbirds (Agelaius phoeniceus) and northern cardinals (Cardinalis cardinalis), he said. Another larger species, the wood thrush (Hylocichla mustelina), has suffered a severe decline over the past couple of decades partly because of cowbird parasitism. So findings that give further insight into how female cowbirds decide which nests to put their eggs in could inform the management of at-risk birds, Merrill said.

Julia John is a science writer at The Wildlife Society. Contact her at jjohn@wildlife.org with any questions or comments about her article.

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