Number of Eggs in a Nest Depends on Climate

Dec. 9, 2008 -- 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.

Related Content:

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.

In 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.