Song Sparrows have social pairs but the females often mate with other males as well. One hypothesis was that by mating with males outside the bonded social pair it would increase the chances of the female’s genes being carried to future generations by mating with a male of higher genetic quality.
New research reported in Science Daily that the opposite is true. Researchers doing a 20 year study of song sparrows found the opposite to be true. Researchers found that offspring from fathered by males outside the bonded social pair had 40 percent less offspring of their own and 30 percent less grandoffspring than offspring within the bonded pair.
Read more at Science Daily For monogamous sparrows, it doesn’t pay to stray but they do it anyway.