Behavior: Brown-headed Cowbirds

They say that only 3% of Brown-headed Cowbird eggs laid make it to adulthood. That may seem surprising low given the number of cowbirds that are around, but not all eggs are laid in the nest of an appropriate host species.

A female cowbird (above) is capable of producing as many as 40 eggs in a season but will only lay one egg in a hosts nest, usually evicting one egg in the process. A second cowbird may lay an egg in that nest. About a third of host nests hold at least two cowbird eggs! Brown-headed Cowbirds have been known to parasitize as many at 220 species; successfully with over 140 of those species. You may be interested to know that eggs have been laid in the nests of Upland Sandpiper, Wilson’s Phalarope, Ferruginous Hawk, California Gull, Blue-winged Teal and Ruby-throated Hummingbird. Quite a diverse group, but really these hosts are not adequate foster parents! The main hosts are Yellow Warblers, as was the case with the nestlings (see Kevin’s May 31st blog) that we saw on the last PA Young Birders field trip, Song Sparrows and Red-winged Blackbirds.

Some ‘hosts’ recognize the cowbirds eggs and will either abandon the nest, cover it over and rebuild, or destroy the egg. There are about a dozen species that fall into the last category and these include Gray Catbird, Baltimore Oriole, Cedar Waxwing, Eastern Kingbird, American Robin, Blue Jay and Brown Thrashers. While it does not happen every time these species have caught on to the cowbird’s ways and will reject by destroying or ejecting the eggs about 90% of the time.