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Peace of the Gods

Features May 2007: Volume 4, Number 2



Why do some birds lay only one egg and other species lay 10 or more?
UC San Diego biologist Walter Jetz sought an answer in a global study of the
number of offspring, or “clutch size.”

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

But Jetz and his colleagues say clutch sizes can also vary widely between closely related species due to variations in their environment, nutrition, health and predation. The results of their study, based on the clutch sizes of 5,290 species of birds, suggest that the kind of nest and the environment in which the bird lives also play a big role in determining how many eggs it lays.

 “Species in seasonal environments, especially those living at northern latitudes, have larger clutches than tropical birds,” says Jetz. He believes the study could ­assist efforts to protect these birds as global warming affects species worldwide.

Contributors to Making Waves: Mario C. Aguilera, ’89, Christine Clark, ’06, Dolores Davies, Caitlin Denham, Raymond Hardie, Kim McDonald

Photography: Munro County Library, Loren McClenahan, Mandeville Special Collections and Scripps Institute of Oceanography