Samplings: September 2009

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Wings of several bird species show different molting patterns.

Sievert Rohwer

To avoid getting grounded, birds must periodically molt, shedding and regrowing worn-out feathers. Once or twice a year small birds quickly replace all their wing quills—the long feathers used in flight. Larger birds, however, can take as long as three years to complete a molt. To find out why, Sievert Rohwer of the University of Washington in Seattle and three colleagues analyzed the relationship of body mass to the length and the growth rate of wing quills in forty-three bird species.

Large birds’ wing quills are longer, in much the same proportion to body size as those of smaller birds. Yet the team found that they take disproportionately long to grow. Therefore, a hypothetical very large bird would take so long to molt that its wing quills would wear out before they could fully grow in. Thus molting may put an unforeseen limit on bird size, the researchers say.

Apparently to compensate for their slow-growing wing quills, some big birds—especially those that must fly to find food or escape predators—shed and replace a few at a time, in a pattern that avoids large gaps. Those that can do without flying for a stretch, such as swans and geese, tend to molt their wing quills all at once.

The authors think that Argentavis magnificens, a long-extinct raptor that weighed a massive 150 pounds, and whose longest wing quills measured five feet, might have had to undergo an extreme version of the latter strategy. They speculate that every few years, it spent about two months sheltering in Argentine cliffs, consuming its fat reserves while molting. (PLoS Biology)