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Weighty Issues

Staff scientist Wesley Hochachka explains new results showing how life among strangers may affect a bird?s weight and well-being

Birds face the same weight-related issues as humans: being either too heavy or too light is bad. However, unlike people, birds can lose and gain 5 percent or more of their total weight in mere hours, due to their high metabolic rates. For small birds, particularly in winter in colder climates, this high rate of weight loss presents a critical problem. If, for example, a chickadee does not gain enough weight before nightfall, it might starve to death before morning. Conversely, carrying too much weight during the day makes a small bird less maneuverable and thus more likely to be captured by a predator.

In the past decade, researchers have shown that birds are keenly aware of the need to steer a careful course between being overweight and underweight. A recent addition to this literature comes from Sweden where two researchers from Stockholm University, Henrik Lange and Olof Leimar, showed that birds even adjust their weights according to whether they are in a familiar or unfamiliar social setting (Behavioral Ecology, July 2004).

These researchers created several small groups of Great Tits (Parus major, relatives of chickadees) in captivity. After the birds had sorted out their pecking orders, clear differences emerged in the amount of weight that dominant and subordinate birds put on during the day. Dominant birds were always heavier in captivity, even though dominance could not be predicted by weight at capture. Thus, the higher weight was the result of dominance and not the other way around.

The most striking result of this study was that when the dominant individuals were moved to new flocks, the subordinate birds in these new flocks altered their daily weight changes, ending the day at a higher weight (thus risking increased chances of predation) and starting the morning at a lower weight (suggesting a closer brush with starvation). The authors suggest that this result could be explained if stress levels made it less likely for the birds to go into torpor at night?a standard energy-saving strategy?after the strange bird was introduced.

These results show that even something as simple as being in a familiar social network can increase a bird?s probability of surviving the winter.

?Wesley Hochachka


For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email:

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