No. 93: May-Jun 1994
Zoologists have been taking it for granted that birds evolved from warm-blooded, active dinosaurs. They may now have to redraw that part of the avian family tree, because the microscopic structure of the leg bones of two species of long-extinct birds suggest otherwise.
"Cross sections of the bones of these birds, which lived during the time of the dinosaurs, reveal growth rings -- concentric rings where normal bone growth was interrupted, possibly because of seasonal temperature changes. No such rings are found in the bones of modern birds, which maintain their body temperatures metabolically even in cold weather. But growth rings are found in such reptiles as crocodiles, which cannot maintain their temperatures metabolically, and in some fossil dinosaurs."
(Browne, Malcolm W.; "Study May Shake Birds Down from the Dinosaur Tree," New York Times, March 17, 1994. Cr. J. Covey.)