News Release

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

Peer-Reviewed Publication

American Association for the Advancement of Science (AAAS)

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.



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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 researchers also saw fewer but bulkier meal deliveries. In other words, Martin says, "Nest predation constrained food delivery and clutch size."

When researchers compared overall results from Argentina with those from Arizona, however, some aspects of their data didn't fit well with the Skutch theory. In Argentina, for instance, predators were far less troublesome, and female birds brought food to their young faster. But, they had smaller clutch sizes in comparison with the Arizona bird mothers. According to the Skutch hypothesis, researchers should have seen higher predation rates and smaller, less frequent feedings among the South American bird families.

"In Argentina, we were monitoring clutches with roughly half the number of nestlings that we saw in Arizona, and yet, the parents were bringing food to them at higher rates," Martin reports.

Clearly, the Science paper concludes, smaller clutch sizes in southern areas can't be explained solely by food delivery or predation rates. Martin and his colleagues recommend building on the Skutch theory by assessing parent mortality rates in northern and southern climates. Birds who must endure cold winters or long, arduous migrations may need larger clutches to ensure the survival of their species, he speculates.

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