BUFFALO, N.Y. -- Male and female black-throated blue warblers, a bird species common in the northeast U.S., have a reputation for practicing monogamy and sharing in the raising of their young.
However, the mere presence of a fertile female in the nest next door can be enough to cause a male to stray, leaving his female mate unguarded, according to a paper scheduled for publication in the July issue of Auk, the journal of the American Ornithologists' Union, authored by Helen C. Chuang, a doctoral candidate at the University at Buffalo, and Michael S. Webster, Ph.D., UB assistant professor of biological sciences.
Although previous studies have shown that otherwise monogamous bird species often do have extra-pair young in their nests -- birds that are not fathered by the companion mate of the mother -- this is the first time that biologists have identified a reason for the phenomenon: local breeding synchrony.
"These birds are socially monogamous but genetically promiscuous," explained Webster.
While this is a puzzling scenario, he said it often is the case with many other birds and mammals, including humans.
"In this case, if a male has several neighboring females that are egg-laying at the same time that his mate is, then the male is more likely to have extra-pair young in his nest," explained Webster, pointing out that among birds, fertilization usually takes place during the time that the female is laying her eggs.
"Our interpretation of these results is that, faced with the opportunity to mate with neighboring females who are building nests and laying eggs, the male spends some time pursuing matings with these females and does not guard his own mate from the copulation attempts of other males," said Webster.
That, in turn, makes the mates of the straying males that much more vulnerable to attention from other roaming males, thereby escalating the chances that the male also will find extra-pair young in his nest when he returns.