JUNE 24,
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.
"Warblers
can't tell the difference between their own offspring and cowbirds," said
Wendy Schelsky, a principal scientist at the Illinois Natural History Survey
and co-author of the study. "They do a really good job of raising
cowbirds, even though cowbird chicks are larger and need more food."
The
researchers studied the interactions between cowbirds and warblers for seven
years to determine whether there was a difference in the relative number of
males and females among cowbird offspring. They collected DNA samples from
cowbird eggs or newly hatched chicks.
"Other
scientists have not seen any difference in the sex ratios of brood-parasitic
birds," said study co-author Mark Hauber, a professor of evolution,
ecology, and behavior at the University of Illinois at Urbana-Champaign.
"This is the first time anyone has detected a seasonal bias and we believe
that it is due to our large sample sizes."
More
female than male cowbirds are hatched early in the nesting season, and the
pattern is reversed in July, new research finds. An adult female cowbird, upper
right, perches on tree stems above an adult male cowbird. Credit: Photo (c) by
Michael Jeffords and Sue Post
The
researchers think their results may reflect the different developmental
trajectories of male and female cowbirds.
