Cowbirds Change Sex Ratio of Eggs Based on When Eggs Are Laid in Breeding Season; More Female Eggs Laid Early; More Male Eggs Laid Late; Large Sample Size May Have Allowed Scientists to Detect Seasonal Sex Bias for First Time in Brood-Parasite Birds

[This article was written by Ananya Sen, a graduate student in Microbiology at the University of Illinois at Urbana-Champaign. Ms. Sen is also a science writer and her articles can be found at This article was originally published as a Research News article by the University of Illinois News Bureau ( Permission to reprint this article in BioQuick News has also been granted by Ms. Sen and approved by University of Illinois New Bureau Editor Diana Yates.] 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 so-called “brood parasites.” They lay their eggs in the nests of other birds and let those birds raise the cowbird young. Prothonotary warblers (see photo and name explanation below) are a common host of cowbirds. “Warblers can’t tell the difference between their own offspring and cowbirds,” said Wendy Schelsky, PhD, (, a Principal Scientist at the Illinois Natural History Survey (INHS) and co-author of the study, which was published online on June 2, 2020 in the Journal of Avian Biology ( The study is titled “A Seasonal Shift in Offspring Sex Ratio of the Brood Parasitic Brown‐Headed Cowbird (Molothrus ater).” “They do a really good job of raising cowbirds, even though cowbird chicks are larger and need more food.” Dr. Schelsky uses molecular tools and ecological experiments to study life-history traits related to predation and parasitism.
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