Blue Tit Birds Slow Biological Clock by Choosing Best Mates

Ornithological cuckoldry has puzzled biologists before, particularly when it comes to the blue tit. Scientists have now discovered a documented link between a feathered female's fertility and her choice of a mate.

In other words, picking a presumably perfect partner may improve a hopeful mother's chances of extending her years of reproductive capabilities - at least in the bird kingdom.

What have bird scientists discovered about blue tit fertility and mate selection?

Ornithologists at the National Evolutionary Synthesis Center in Durham, North Carolina, conducted a study of blue tit fertility, announcing the results in the "Oikos" scientific journal in January 2011.

Scientists examined data from 40-year studies of blue tits on the French Mediterranean island of Corsica. Bird researchers had tagged blue tits with ankle bands to monitor their mating habits, count eggs the females produced and observe the development of their offspring. The studies tracked nearly 1,200 blue tits for decades.

The blue tit fertility study spawned several clear observations.

First, bird fertility decreases with age, just like it does in humans and other creatures. Female blue tits lay fewer eggs as they age, and they tend to do so later in the season as well, when food may be less abundant for their offspring. Thus, survival rates may drop for these unfortunate baby birds.

Second, a key factor in the longevity of female blue tit fertility was found to be largely dependent upon the quality of the male mates they select. Most specifically, the males' reproductive history was a critical component in the females' long-term fertility.

"The 'history' of the male matters," explained Anne Charmantier of France's National Center for Scientific Research, a co-author of the blue tit fertility study. Apparently, male blue tits with the earliest paternal experience were optimum partners for females with the longest fertility.

"Females that repeatedly pair with early-reproducing males are better off," reported Josh Auld, of the National Evolutionary Synthesis Center and co-author of the bird study. "They don't age as fast."

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

At least for blue tits, then, males have ticking biological clocks as well. Maybe it's time to put the "men" in "menopause" - at least for blue tit birds.

How do female blue tits choose their mates?

Apparently, female blue tits are strongly influenced by certain qualities in their existing and prospective mates, such as plumage brightness, song performance and physical appearance.

"These results are very unexpected, and one cannot help but wonder if they apply to vertebrates more generally," commented Saran Twombly, program director in the National Science Foundation's Division of Environmental Biology, which funded the blue tit fertility research. "The work ... emphasizes the need to integrate physiology, behavior, and other biological disciplines to understand what organisms do and how they interact in the wild."

What is a blue tit?

The blue tit (cyanistes caeruleus ogliastrae) is a small blue and yellow woodland bird that lives in the Northern Hemisphere. Feeding primarily on insects (particularly caterpillars), grains and nuts, the blue tit is frequently seen in cultivated gardens and parks in Europe and North America.

With an average life expectancy of one to two years, the agile blue tits do not migrate, tending to remain in their original habitats year-round.

Generally regarded as socially monogamous, at least annually, blue tits have been observed to pursue extra-pair matings (ornithological adultery). Lately, bird biologists have begun calling these extra couplings adaptive behaviors, marked by blue tits' instinctive yearning to optimize reproduction of the species.

Most often, the strongest female blue tits tend to seek extra male partners, particularly if they have paired with weaker males initially. The female birds who initially settled with substandard male partners were most likely to participate in extra-pair matings. This ornithological observation seems to support the popular scientific hypothesis of good gene selection.