New Science study revisits the debate over bird parents, predators, and family planning

Washington, D.C.-- What factors affect the size of bird families? Understanding the forces that drive evolutionary changes in bird behavior and life history "may help us better assess the vulnerability of different species to extinction or population problems," researcher Thomas E. Martin says of his latest study, appearing February 25 in the journal, Science.

In tropical and southern regions, female birds tend to have fewer babies per nesting attempt, compared to their more prolific counterparts in moderate, northern climates. This curious fact of bird life has fueled an ongoing debate over whether clutch sizes are limited mostly by food supply, by predators, or both.

Researcher Alexander Skutch coupled these two theories in 1949, when he suggested that predators limit the meals available to bird babies by forcing parents to limit the rate at which they visit the nest, ultimately resulting in the evolution of smaller families. After all, bird parents who fly off the nest to find food risk attracting the attention of hungry predators, from squirrels and chipmunks to other birds, explains Martin, a research scientist with the U.S. Geological Survey's Biological Resources Division and a professor at the University of Montana.

Yet, Skutch's longstanding hypothesis had never been put to the test -- until now.

Martin's research team -- including Paul R. Martin, Chris R. Olson, Britt J. Heidinger, and Joseph J. Fontaine of the Montana Cooperative Wildlife Research Unit at the university -- staked out forests in two different regions. Some 1,331 nests were monitored in subtropical Argentina, and then compared with 7,284 nests in Arizona.

As expected, typical clutches were much smaller in Argentina, where bird mothers produced about 2.58 eggs per nesting, versus 4.61 in Arizona. Separate analyses in each region seemed to support the Skutch theory because clutch size was, indeed, smaller for species with higher predation rates, where res

Contact: Ginger Pinholster
American Association for the Advancement of Science

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