Image: Small clutch
Cagan Sekercioglu
The little tinamou usually lays two eggs in a small depression on the forest floor. Ground nesters tend to lay fewer eggs since they are more vulnerable to predation.
updated 12/9/2008 1:34:12 PM ET 2008-12-09T18:34:12

Some birds lay one egg while others lay up to 50 or more, and now researchers have figured out why, to the point where they can accurately predict egg counts for virtually all bird species.

Their forecasting might seem like an egg conjuring trick, but the secret comes down to predictors, such as the type of nest the bird builds and how close each avian species is to the poles.

Tropical birds, as it turns out, have a more laid-back approach, with generally fewer eggs within each clutch.

"You'd think it would be just the opposite, as the hypothesis for years was that the large amount of resources in the tropics would lead to more egg laying productivity, but we determined that wasn't the case," lead author Walter Jetz told Discovery News.

"Seasonality is far more important than the absolute amount of resources," added Jetz, an associate professor of biology at the University of California at San Diego.

He explained that the closer birds are to the poles, the more extreme seasons they experience. Species in these locations often have higher mortality rates. It's therefore to their advantage to lay more eggs when they can, before it's too late.

Defined seasons also lead to peak periods of food and habitat availability, such as during spring. Suddenly birds find themselves with "huge amounts of resources, so they capitalize on them much more than at the tropics, where no such peaks exist," Jetz said.

For the study, he and his colleagues compiled information on clutch size, avian body mass, migratory behavior, nest type, bird diet, latitude, temperature and more. The scientists observed patterns in the numbers of eggs birds laid, allowing them to create a model that predicts variations in clutch sizes both on global and more localized levels.

The findings are published in the latest PLoS Biology.

Secret mammal singersIn addition to seasonality, whether or not a bird lays its eggs in a protective cavity or just out in the open helps to predict its number of eggs.

"The hypothesis is that cavities are better protected, so woodpeckers, chickadees and other cavity nesters are less likely to be raided, so they tend to invest more in egg laying," Jetz said.

"Ground nesters, on the other hand, shouldn't put all of their eggs in one basket," he added, since predators are then more likely to find themselves with a free and easy egg dinner.

It would seem logical that all birds would hide their eggs in protective cavities, but he explained that, "not all the world is covered with trees." Birds in the desert, for example, often have no choice but to lay their eggs more in the open. Cavity nesting also is an ability that only a select group of birds evolved.

The scientists additionally determined that migrating birds tend to lay more eggs. Birds within certain families also tend to share similar clutch sizes. Big, flightless birds, for example, may often have large clutches containing anywhere from 10 to 74 eggs.

Robert Ricklefs, a professor of biology at University of Missouri at St. Louis, told Discovery News that, "Dr. Jetz's study is notable for its comprehensiveness and the excellent analytical applications."

Ricklefs added that he was "especially gratified to see that seasonality of temperature, migration, and nest type play the dominant roles, as these factors have been particularly prominent in the literature."

Given the importance of climate to birds, the global egg count, and consequently avian populations worldwide, may be in jeopardy due to climate change.

It has the potential of not only harming a "bird's way of life, but also its where of life," Jetz said, referring to how many species may be forced to shift their geographical ranges.

In the future, the new clutch size prediction model could help conservationists and other avian experts to better understand bird egg laying patterns in what Jetz and his colleagues call "a world of change."

More on   Birds Climate

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