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Wendy Schelsky

Cowbirds change their eggs’ sex ratio based on breeding time

June 25, 2020

Brown-headed cowbirds show a bias in the sex ratio of their offspring depending on the time of the breeding season, researchers report in a new study. More female than male offspring hatch early in the breeding season in May, and more male hatchlings emerge in July.

Cowbirds are brood parasites: They lay their eggs in the nests of other birds and let those birds raise their young. Prothonotary warblers are a common host of cowbirds.

June 25, 2020

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