David Harwood sent us these photos of a Pine Warbler feeding a young Brown-headed Cowbird that it no doubt raised.  Brown-headed Cowbirds lay their eggs in the nests of other species, including small warblers like this Pine Warbler.   They then leave it to the new foster parents to raise the young cowbird.

Pine Warbler and young Brown-headed Cowbird

Pine Warbler and young Brown-headed Cowbird

Young Brown-headed Cowbird

Young Brown-headed Cowbird

Pine Warbler feeding young Brown-headed Cowbird.

Pine Warbler feeding young Brown-headed Cowbird.

One theory is that Brown-headed Cowbirds evolved to follow the buffalo herds to feed on the insects the buffalo scared up.  Since the buffalo roamed the plains and the cowbirds chose to follow them there was no time for nest building and egg laying.  The cowbirds responded by laying their eggs of in the nest of other species.

Now lawnmowers have replaced the buffalo and the cowbirds have become more widespread and parasitize the nest of more species.   Cowbird eggs usually hatch one day ahead of the host’s eggs. In addition cowbird nestlings usually are larger and grow faster than the host’s young, which enable them to garner more than their fair share of the food brought to the nest.  The result is the young of the host are often ejected from the nest or starve.

Cowbird parasitism played a major role in the fall of  of Kirtland’s Warbler populations.

Here are a few questions and answers about the bully-bird behavior of some cowbirds.

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