Why Do Bird Species Lay Different Number of Eggs?


All photos by Cagan Sekercioglu

Why do some bird species lay only one egg in their nest, and others ten?

The substantial variation in number of eggs in the nest (clutch size) between bird species has long puzzled behavioral, ecological, and evolutionary biologists. One method to explain it focused on the biology of species, such as body weight. Another approach looked at the environment, such as seasonality.

By using data on clutch size for 5,290 species, and combining it with a wealth of information on the biology and the environment of these species, scientists believe they may have some answers.


“With this approach, we were able to explain a major proportion of the global variation in clutch size and also to predict with high confidence the average clutch size for types of birds living and breeding in certain environments,” said Walter Jetz, an associate professor of biology at University of California San Diego and the senior author of the study.

“For example, cavity nesters, such as woodpeckers, have larger clutches than open-nesting species. And species in seasonal environments, especially those living at northern latitudes, have larger clutches than tropical birds,” he said in a statement.

A paper on the research is published in the current issue of the scholarly journal PLoS Biology. Two other authors were Cagan H. Sekercioglu, Center for Conservation Biology, Department of Biology, Stanford University, California, and Katrin Bohning-Gaese, of the Institut fur Zoologie, Abteilung Okologie, Johannes Gutenberg-Universitat, Mainz, Germany.

Clutch size in birds and reptiles has long been studied by biologists, who have found generally that species that are short-lived or have a low survival rate among their offspring tend to lay more eggs at one time to increase the chances of having surviving offspring, according to a UC San Diego news release about the research.


“In contrast, longer-lived species or those with a higher survival rate among offspring tend to lay fewer eggs in their nests and invest more time and effort in raising their offspring.

“However, the reasons why one species of bird may lay one egg and another 10 are more complex because clutch sizes can vary widely between closely related species due to variations in their environment, nutrition, health and predation.”

The researchers were aided in their investigation by detailed records of the life histories of birds throughout the world.

“Humanity’s fascination with birds since the dawn of time has meant that thousands of ornithologists and millions of bird enthusiasts have been collecting detailed data about birds’ lives for centuries,” Sekercioglu said. “Published in countless books, popular and scientific articles, these data have made birds the best known major group of organisms.”

“In this study,” he added, “we answer one of the most basic questions asked about birds: Why do bird species lay different numbers of eggs?

“The integration of geographic and life history datasets enabled us to simultaneously address the importance of ecological, evolutionary, behavioral and environmental variables in shaping the clutch size of the world’s birds.”


The research shows that increased environmental variation causes birds to lay larger clutches, Sekercioglu said.

Most ornithological research has taken place in the highly seasonal environments of North America and Europe, but most bird species live in less seasonal tropics. Therefore, the small clutch size seen in less-studied tropical birds is the norm, not the exception.”

Increased predation pressure experienced by open-nesting birds also causes them to lay smaller clutches than cavity-nesting birds, literally having fewer eggs in one basket to spread the risk, Sekercioglu said.

The three biologists said they believe this information will become increasingly more important in efforts to protect these birds as rapid environmental changes due to global warming affect these species.

“Our results demonstrate not only where bird species live, but how the way they live their lives, specifically their reproduction strategies, has evolved in close association with climate, particularly seasonality,” said Jetz of UC San Diego. “Rapid changes to the global geography of climate are likely to impact both aspects and to potentially perturb the long-evolved link between the ‘where’ of life and the ‘way’ of life in many species.”

Sekercioglu added: “The majority of bird species live in the tropics. Tropical birds’ smaller clutch size is greatly shaped by more stable climates and these birds’ survival depends on the continuity of the weather conditions they have adapted to during millennia.

“Climate change and a potential increase in climatic fluctuations in the tropics may make these birds highly vulnerable. Hundreds of tropical bird species are already threatened with extinction and a potential clash between changing climate and their reproductive strategies may cause additional harm.”


All photos by Cagan Sekercioglu. For more photos and captions to these and the other photos visit the photo gallery that accompanied this research on the UC San Diego Web site.

Sekercioglu has received research funding from the National Geographic Society for a non-related project. Some of his other research is referenced in the National Geographic News story Are Birds Best Hope for Pest-Ridden Coffee Crops?

Related National Geographic News stories:

1/4 of U.S. Birds at Risk, Study Says

Early Birds: Is Warming Changing U.K. Breeding Season?

Are Some Birds Not Flying South Due to Warming?

Changing Planet

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