Sievert Rohwer at the University of Washington in Seattle and his colleagues studied 43 species of bird, assessing size, the length of flight feathers and timing of moulting cycles.
Flight-feather length is proportional, they say, to body mass raised to the one-third power, so that feather length roughly doubles with a tenfold increase in a bird's weight. But feather growth rate is proportional to body mass raised only to the one-sixth power. The trade-offs in time and energy required to replace long feathers therefore limit maximum bird size.
