A study published in American Naturalist found that song sparrows who have flings outside their monogamous relationships conceived young with low fertility success. This opposes the idea that the reason animals stray is to increase reproductive success. Researchers followed Song Sparrows for twenty years. In Science Daily the lead scientist said,

These results are remarkable because they are completely opposite to expectation... They show that females suffer a cost of promiscuity because they produce worse offspring through extra pair mating. Rather than answering the question of why females should mate promiscuously, [these results] have blown the question wide open.

(via Science Daily)

(Song Sparrow, San Mateo by Peter LaTourrette)