There is no doubt about it — Cowbirds exhibit ingenious methods of manipulating other bird species into raising their little brown-headed offspring. Cowbird eggs have been found in the nests of 220 bird species, ranging in size from hummingbirds to raptors.
By watching the nesting activities of particular target bird species, Brown-headed Cowbirds time their reproduction with that of their host, forcing the host to raise their chick for them. A single female, laying just one egg in a host nest, may produce up to 36 eggs in a breeding season.
Cowbird chicks are usually larger than the host species chicks, hatch a little earlier, and grow a little faster, so they may consume the bulk of the food that parents bring to the nest, to the detriment of the host’s own chicks.
The end result is a single large Cowbird chick that follows its host parent (in this case, a Chipping Sparrow) around incessantly begging for food.
Some species, such as Robins, Gray Catbirds, and Brown Thrashers, physically eject cowbird eggs from their nest and seem less susceptible to nest parasitism. Blue-gray Gnatchatchers desert their nest if they find cowbird eggs in it and Yellow Warblers simply build another nest on top of the one that was parasitized, hoping the cowbirds don’t find them again.
However, Cowbirds have learned to retaliate against some egg rejectors with “mafia-like” behavior.
Ever watchful, Cowbirds not only monitor the nesting activity of the host, but watch what becomes of their eggs in the host nest. If the host rejects the foreign egg, cowbirds return to the nest and destroy the host eggs!
And the “mafia” behavior works. Prothonotary Warblers that rejected cowbird eggs managed to raise only one chick because cowbirds punctured the other eggs or threw the host’s offspring out of the nest, Those that tolerated the presence of a cowbird chick in their nest raised three of their own chicks. So, it pays to be tolerant of a nest parasite if you’re a Prothonotary Warbler.
Cowbirds have even learned to “farm” their host species by manipulating the hatch time of host eggs to match their own chick’s hatch time. This is achieved simply by destroying those host eggs in the nest laid earlier than their own.
Who knew that becoming an accomplished nest parasite required such intelligence?