High metabolic rate

Another basic shared characteristic:

http://en.wikipedia.org/wiki/Bird
"Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton."

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0004497
"The relatively high efficiency of flow-through ventilation was likely one of the key developments in pterosaur evolution, providing them with the respiratory and metabolic potential for active flapping flight and colonization of the Late Triassic skies. This interpretation is consistent with other lines of evidence supporting relatively high metabolic rates in pterosaurs, including the filamentous nature of the integument [e.g. 16], [45], [46], a flight performance comparable to that of extant birds and bats [1], [3], [4], [6], [7], [16], [17] and relatively large brain size [47]."
(Leon P. A. M. Claessens^1*, Patrick M. O'Connor², David M. Unwin³⁾