Feather growth limits size of flying birds
Time required for moulting may be more important factor than weight.
The size of flighted birds is limited by the demands of keeping their feathers in good condition, and not simply by the effort needed to keep them up in the air.
Birds' feathers can be damaged by the physical rigours of flight, parasititic and bacterial infections, and exposure to ultraviolet light. It's crucial for birds to periodically replace damaged feathers, but this requires time and energy that could otherwise be used to find mates, rear chicks or migrate.
Sievert Rohwer at the University of Washington in Seattle and his colleagues studied 43 species of bird to assess the relationship between a bird's size, the length of its flight feathers and the time it takes to grow new feathers. They report their findings in PLoS Biology1.
The team showed that the length of a bird's flight feathers is proportional to its body mass raised to the one-third power, so that feather length roughly doubles with a tenfold increase in the bird's weight. But feather growth rate is proportional to body mass raised only to the one-sixth power.
"So as birds get bigger, the rate of feather growth fails to keep up with the increase in flight-feather length, forcing larger birds to spend disproportionately more time growing their flight feathers," says Rohwer.
Most small birds regrow all of their primary flight feathers at least once a year, replacing the 9 or 10 primary feathers per wing sequentially. Rohwer and his co-workers suggest that the trade-offs involved in regrowing feathers place an upper limit of about 3 kilograms on birds that moult in this way.
Above 3 kilograms, the time required to grow new feathers makes this strategy too time-consuming, having a detrimental effect on breeding and migration.
Many larger birds, such as albatrosses, get around this by stretching the moult over two to three years. This ensures that a smaller fraction of each year is spent moulting. Other species replace several feathers at the same time, or replace them all simultaneously and temporarily forgo flight.
But each alternative imposes a cost. Replacing more than one flight feather at a time reduces the aerodynamic efficiency of flight; replacing them all at once leaves birds temporarily flightless, making feeding and escaping predators more difficult.
These tradeoffs also raise questions about how the largest flying bird ever discovered — Argentavis magnificens, which lived around 6 million years ago and weighed roughly 70 kg — could have coped with feather replacement. Rohwer and his colleagues speculate that A. magnificens solved the problem of replacing large, slow-growing feathers by fattening up and then going in for one big, long moult, as geese and swans do today.
Size is still an important factor, points out Anders Hedenström of Lund University in Sweden. Aerodynamics suggests an upper weight limit of about 15 kilograms for birds whose flight is powered principally by flapping their wings. Soaring birds can achieve a much greater mass — for instance, some bustards weigh as much as 20 kilograms. A. magnificens was probably such a bird.
Yet many birds do not achieve the maximum size permitted by their mode of flight, says Rohwer, probably because of the time demands of feather replacement.
- Rohwer, S., Ricklefs, R. E., Rohwer, V. G. & Copple, M. M. PLoS Biol. 7, e1000132 (2009).