The size of flighted birds is limited by the demands of feather maintenance.
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.
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For a longer story on this research, see http://tinyurl.com/l3ge9b.