Many studies on avian microanatomy have established a relationship between high bone compactness (i.e., considerable degree of osteosclerosis) and diving behavior. Greatest degrees of compactness have been observed in the femora and humeri of fossil and extant penguins, femora of Hesperornis Marsh, and Polarornis gregorii Chatterjee, and to a somewhat lesser degree, in the humeri of flightless Pan-Alcidae. Within Anatidae, humeral and femoral compactness among diving species is greater than among non-diving ones, whereas flightless diving species have a greater degree of compactness than their volant close relatives. In Cayaoa bruneti Tonni, an extinct flightless foot-propelled, diving anatid with extreme forelimb reduction, femoral osteosclerosis is as great as that of penguins. Osteosclerosis in the femur of both foot- and wing-propelled divers could be part of the consequences of flightlessness and a result of selection to counter buoyancy.