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A lot of the 'colorful" song birds (bluebirds for example) are having a hard time keeping their numbers up because of the cowbird.
The cowbird will lay it's egg in with the bluebirds, and it will hatch first.
The chick will then proceed to push out its competitors, and I mean literally push them out of the nest. That is why when driving through a lot of rural areas they put up bluebird boxes to try and prevent the cowbirds from invading the nests.
I saw a special on this a number of years ago, but I can't recall where.
(Here's another screw-up that';s man-made: snakes in Hawaii.
Hawaii doesn't have snakes, except for the ones that stowed away with air shipments over the years. The result is that there are species of birds in Hawaii and the south pacific that have been eradicated.)
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