"The female cowbird sneaks around the forest, laying her egg in other species' nests, " Stutchbury said.

WSJ: Northeast drilling boom threatens forest wildlife

Nests from which cowbird eggs had been removed, but which lacked protection, did badly.

ECONOMIST: How cowbirds run protection rackets

In the following seasons Dr Hoover and Dr Robinson removed cowbird eggs from some of the parasitised nests.

ECONOMIST: How cowbirds run protection rackets

Nests thus attacked yielded, on average, but a single fledgling, whereas those with a cowbird egg in them yielded three warbler fledglings.

ECONOMIST: How cowbirds run protection rackets

Warblers whose nests were thus protected did well, raising an average of four chicks to maturity in the absence of a cowbird parasite.

ECONOMIST: How cowbirds run protection rackets

The cowbirds' dastardly tricks do not stop at this protection racket, either, for a fifth of those warbler nests that had never had cowbird eggs in them also got destroyed.

ECONOMIST: How cowbirds run protection rackets

Forest fragmentation opens the door to invasive species such as the cowbird, a type of blackbird that normally prefers open land, said Bridget Stutchbury, a biologist at York University in Toronto who studies forest songbirds.

WSJ: Northeast drilling boom threatens forest wildlife

If a cowbird female fails to lay in a warbler nest in time for her egg to hatch with those of the host, she can reset the clock in her favour by killing the first clutch.

ECONOMIST: How cowbirds run protection rackets

Paying protection money in the form of food for the cowbird nestling thus looks a good deal from the warbler's point of view, and explains why cowbirds do not need to disguise their eggs to look like those of prothonotaries.

ECONOMIST: How cowbirds run protection rackets