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Could Big Bird fly? Answer may lie in the feathers

The larger the bird, the more complex the molting process, which may hinder flying, researchers find.

June 20, 2009|Shara Yurkiewicz

What determines how big a flying bird can be? The answer, in part, is the time it takes for the creatures to replace their feathers, researchers have found.

The study, published Tuesday in the journal PLoS Biology, found that the larger the bird, the more challenging it is for the growth of new feathers to keep up with feather length. At bigger bird sizes, feathers wear out before new ones can grow.

It is even possible that feathers may dictate an upper limit for the size of flying birds, although that is not proven, said study coauthor Robert E. Ricklefs of the University of Missouri-St. Louis.

For birds to fly, their feathers must be in good condition. But continued abrasion and exposure to air, bacteria and ultraviolet light causes their gradual deterioration. Worn-out feathers must be shed and replaced in a process called molting.

But losing wing feathers hinders flying ability. For birds that depend on flight to find food and mates and evade predators, the process makes the animal vulnerable. So molting must occur in a way that minimizes flying problems.

Sievert Rohwer of the University of Washington and colleagues examined specimens of 43 bird species, comparing body size, length of flight feathers and their growth rate.

Larger birds have longer feathers: Length roughly doubles for each tenfold increase in bird mass, the scientists found.

Larger birds also have faster feather growth rates, but not enough to keep up with the larger feather size. It takes about 1.5 times longer to replace a feather with each tenfold increase in bird mass, the authors calculated.

These findings may explain why different birds have evolved different ways of molting, the scientists wrote.

Small birds -- ones that weigh less than 2.2 pounds and need less power to stay in the air -- shed adjacent feathers. This creates a featherless gap on their wing surface, but because of their light weight, they are still able to fly.

Heavier birds wouldn't be able to remain airborne if they had these large, featherless gaps on their wings. So they minimize the size of the gaps by shedding feathers on different parts of the wing or extending the molting process to years instead of months.

For example, California condors, which depend heavily on flight to get food, take two to three years to replace all their long flight feathers.

Species that do not rely on flight for survival, including ducks and geese, replace all their feathers at once.

It is not clear whether feather replacement ultimately limits the size flying birds can reach, Ricklefs said. Swans weighing 33 pounds hold the record. Birds such as ostriches can weigh up to 10 times that, but they are flightless.

But rewind 6 million years, and the extinct Argentavis magnificens of the Miocene Epoch in what is now Argentina clocked in at 154 pounds -- and flew.

Although no one knows how this giant bird molted, the study's authors speculate that it lost all its feathers at once during a period in which it fasted and lived off its fat, much like flightless emperor penguins do today.

"This paper could change the way we think about how different body sizes have evolved in birds," said Eli S. Bridge, a postdoctoral researcher with the Oklahoma Biological Survey at the University of Oklahoma in Norman, who was not involved in the study. The scientists "make it clear that finding time to molt places important constraints."


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