When a bird's eye goes astray

The mere presence of a fertile female in the nest next door may be enough to cause a male black-throated blue warbler to stray.

August 16, 1999
Web posted at: 11:36 a.m. EDT (1536 GMT)

Male and female black-throated blue warblers, a bird species common in the Northeast U.S., have a reputation for being faithful to one another and for sharing the duty of raising of their young. But a bird's eye can wander.

The mere presence of a fertile female in the nest next door may be enough to cause a male to stray, leaving his female mate unguarded, according to researchers at the University of Buffalo.

Although previous studies have shown that otherwise monogamous bird species often do have extra-pair young -- birds that are not fathered by the companion mate of the mother -- in their nests, 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," explains biologist Michael S. Webster. While this may seem like 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.

"When many females are building nests, the entire breeding grounds may be like a big flea market, with many males advertising their wares through song and the females comparison shopping," said Webster.

The biologists identified the extra-pair young through field experiments in New Hampshire and through molecular genetic analysis of DNA taken from blood samples. They found that a relatively high proportion -- about 30 percent -- of the young they observed were fathered by a male who was not their "social" father; that is, not the companion mate of the mother.

Webster explained that when a nest of young was produced at the same time that nests were being produced on neighboring territories, the nest was likely to contain extra-pair young. On the other hand, when a nest was produced in a territory when others were not being produced in the area, the focal nest was unlikely to contain extra-pair young.

The question the researchers now face is whether it is male mate-guarding or female mate choice that is responsible for this pattern.

According to Webster, the male's evolutionary motive for the extra-pair mating can be explained fairly simply. "Mating with more females means more offspring, and in terms of Darwinian fitness, the number of descendant offspring is key," he said.

The trickier issue, he pointed out, is finding out how this benefits the female. "Females could be obtaining 'good genes' by mating with particular extra-pair males -- those that are particularly healthy and have genes making them resistant to disease -- or she could be obtaining direct benefits," he said.

"For example, the extra-pair male may allow the female to forage on his territory once she has copulated with him."

For now, Webster and his co-authors believe that being guarded is actually costly to the female since it prevents her from mating with other males. "Extra-pair copulations and mate-guarding represent a conflict of interests between the male and female in an otherwise cooperative pair," said Webster.

An alternative explanation being tested by the researchers is that breeding synchrony makes it easier for females to compare and contrast potential extra-pair mates.

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