Many Nazca booby females switch mates after successfully raising a chick, according to a Wake Forest University study scheduled for publication in the journal Proceedings of the Royal Society B: Biological Sciences June 13.

This is surprising because there is an advantage to staying together, said Terri Maness, a doctoral student who co-authored the study with David J. Anderson, professor of biology at Wake Forest. The chance of successfully breeding probably improves as the pairs of birds get older and are together longer, as has been found in other birds.

But, often the female seeks a divorce after a few breeding seasons. Since males significantly outnumber females in the colony studied, there are plenty of bachelors available if the female has a wandering eye.

"Our study population has 50 percent more males than females, creating the opportunity for females to trade a current mate, which may be worn-out from recent breeding effort, for a 'refreshed' non-breeding male," Maness said.

It takes a lot of energy to raise a chick and the responsibility is shared by both males and females. They raise one chick at a time. Parents incubate the egg for 43 days. Then, it takes another 100 to 120 days of parental care until the baby bird can fly. The parents will usually continue to provide some meals to the fledgling.

So, if, when mating season rolls around again, a male looks a little ragged from taking care of junior, his mate is likely to choose another. A male's capacity to raise offspring, a quality that can vary with time, may carry more weight with a mate than stellar genes or past breeding success, Maness said.

She used 14 years of data on about 950 males and 700 females in the study. During that time, the majority of the females in this group gambled on a new mate, sometimes one that had never bred before.

This is one of the few studies that addresses divorce in successfully breeding bird pairs, Maness said. "This study really predicts that the probability of divorce increases with the birds' success at breeding and raising a chick, because the effort required may tire out the male and consequently his mate may reject him."

It also shows that males of this species are probably never permanently in or out of the mating game, in contrast to many other animals, she said. A male Nazca booby cast off by a current mate may be selected by a different female the next year, after he has had a chance to regain his condition.

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