Females of the Brown-headed cowbirds (Molothrus ater) fly into other birds’ nests and lay an egg for other birds to hatch and raise. Thus, cowbirds are classified as ‘brood parasitic’ birds—meaning that they lay eggs in the nests of other bird species to be raised by the host. In this way, the cowbird parents do not have to take the time to build nests and raise their young. Rather, they use this time to feed and lay more eggs.

However, Jeffrey P. Hoover, of the Illinois Natural History Survey (Champaign) and Scott K. Robinson (Florida Museum of Natural History (Gainesville) have found that if warblers remove the cowbird eggs from their nests, then the cowbirds return and destroy the warbler eggs in retaliation.

The researchers discovered this behavior after studying the small songbird Prothonotary warblers (Protonotaria citrea) in nesting boxes in southern Illinois for over ten years. After seeing cowbird behavior for that decade, they realized how cowbirds operate: leave their eggs in other birds’ nests to be taken care of.

However, they wondered why these birds didn’t just remove the cowbird eggs from the nests, especially, when the much larger cowbird chicks often times cause the other warbler chicks to starve because they are so demanding for food.

Hoover and Robinson set up an experiment where they removed cowbird eggs from warbler nests. Vandalism of the nests occurred. The warbler eggs were smashed by something unknown. They greased the pole that the nesting box was placed on in order to assure other predators such as snakes could not get to the eggs. They also placed a front (with a small hole) onto the nesting box so that the cowbirds could not enter. And, the vandalism stopped.

The researchers found that when cowbird eggs remained in the nest, only 6% of the warbler eggs were damaged. However, when cowbird eggs were removed, 56% of the warbler eggs were destroyed. No damage occurred when the cowbirds could not get to the warbler eggs.

Hoover and Robinson concluded that the warblers learned their young survived better when they took care of the cowbird chicks. Even though some young warbler chicks died of starvation, more warbler eggs were destroyed in retaliation attacks by cowbirds.

The researchers state this is the first time that gangster-like behavior has been found in birds. Maybe cowbirds should be nicknamed “Don Corleone cowbirds”?

Their results will appear in an article of a future issue of Proceedings of the National Academy of Sciences.

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