Birds are related to T. rex, one of the largest predators ever to stalk the Earth, so why are they so puny by comparison, even the birds that don't fly? It's all about balance, apparently.
Theropod dinosaurs, the ancestor of birds, walked with their legs vertical using their tail for balance. As their descendents evolved to fly, their heavy tails became a liability and shrank to a stub. To keep their balance, the evolving birds began to crouch, with their femurs tilted back to keep their knees under their centre of gravity. They kept this position even after some had ceased to fly, and the extra strain made their femurs stronger.
Nicholas Chan at Macquarie University in Sydney, Australia, plotted the length and circumference of femurs in 43 living and extinct flightless birds against their body size, including the extinct moa and the elephant bird. He then did the same for 81 theropod fossils, and compared the two sets.
The dino femurs grew longer and thicker as body size increased, Chan says. But in birds, there was a "distinct plateau" between 300 and 500 kilograms, with femur length constrained by the need to keep the knee near the centre of gravity. That makes it harder to support a large body, and so it limits their size, argues Chan.
Chan is taking a useful step forward, says John Hutchinson of the Royal Veterinary College at the University of London, but he cautions that muscle strength may be more of a limiting factor on bird size than bone strength.
The work was presented at the Society of Vertebrate Paleontology meeting in Los Angeles earlier this month.
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.