On a late July visit to Washington’s Landing (Herr’s Island) I saw two song sparrow families with begging fledglings. Unfortunately the begging youngsters were brown-headed cowbirds, not song sparrows.
Female brown-headed cowbirds lay their eggs in the nests of smaller birds. Each cowbird chick is raised, not by its own mother, but by foster parents of another species. To make matters worse, cowbird parents lurk near the foster nest to make sure their own baby survives. They remove the host’s eggs or kill the foster parents’ young to give their own chick a better chance.
Cowbirds parasitize many species but are especially fond of song sparrows and yellow warblers. Yellow warblers are well aware of cowbird eggs and will “abandon” the nest by building a new nest on top of the old one. Experienced song sparrows get upset but don’t have an immediate solution.
However, song sparrows have a secret weapon — their breeding season is longer. Their first of two to four broods may begin before cowbirds are ready to lay eggs while the last nest starts after cowbirds are done.
In Pennsylvania brown-headed cowbirds stop laying in early July while song sparrows are still going strong. What I saw at Washington’s Landing was this year’s last round of cowbird babies.
(photo from Wikimedia Commons; click on the caption to see the original)