In birds as in humans, female fertility declines with age.

But some female birds can slow the ticking of their biological clocks by choosing the right mates, according to results of a study published online last week in the journal Oikos.

Female birds become progressively less fertile as age takes its toll, says biologist Josh Auld of the National Evolutionary Synthesis Center in Durham, North Carolina, and co-author of the Oikos paper.

Older females lay fewer eggs, and they lay them later in the season--at a time when less food is available for their chicks.

But despite abundant evidence of fading fertility in females, scientists knew little about the role played by their mates. "The thought was that males didn't matter," Auld says.

But they do.

"These results are very unexpected, and one cannot help but wonder if they apply to vertebrates more generally," says Saran Twombly, program director in the National Science Foundation's Division of Environmental Biology, which funded the research.

"The work highlights the power of data synthesis to reveal patterns that arise only by combining lots of data to address new questions," says Twombly.

"It also emphasizes the need to integrate physiology, behavior, and other biological disciplines to understand what organisms do and how they interact in the wild."

To find out if males factor into female fertility, biologists took advantage of long-term data collected on birds known as blue tits.

Since the late 1970s, scientists have studied thousands of these blue-and-yellow forest birds on the French island of Corsica.

Blue tits breed once a year, often with a different mate each time.

By attaching identification bands to the birds' ankles and monitoring their nests, scientists were able to keep track of who mated with whom, how many eggs they laid and when, and how the fledglings fared over time.

When the authors analyzed data for nearly 600 females and 600 males from 1979 to 2007, they found a surprise: how fast a female's fertility fades depends partly on her partners.

The important thing for females is not the age or identity of her mates, but her partners' paternal past.

"The 'history' of the male matters," says paper co-author Anne Charmantier of France's National Center for Scientific Research (CNRS).

Fertility declined less quickly for females whose mates became first-time fathers young.

"Females that repeatedly pair with early-reproducing males are better off," says Auld. "They don't age as fast."

Males who got a head start on fatherhood--within their first year of life--may be healthier or more experienced mates than dads that delayed breeding.

"Males that start breeding early may be in better condition, or have a lower parasite load," Charmantier says.

Healthier or more experienced males may also be better partners in parenting, such that time takes less of a toll on mom.

"The male helps the female build the nest, and he brings her food while she's laying and incubating the eggs," says Auld. "He also helps care for the hatchlings."

Surprisingly, males play a role in how fast females pass their prime.