Seven species of ground-dwelling birds (body mass range: 0.045 to 90 kg) were filmed while walking and running on a treadmill. High-speed light films were also taken of humans to compare kinematic patterns of avian versus human bipedalism. Consistent patterns of stride frequency, stride length, step length, duty factor and limb excursion were observed in all species, with most of the variation among species being due to differences in body size. In general, smaller bipeds have higher stride frequencies (aM^-0.18), shorter stride lengths (aM^-0.38), and more limited ranges of speed within each gait than large bipeds. After normalizing for size (based on Froude number, after Alexander, 1977), remaining kinematic variationis largely due to interspecific differences in posture and relative limb segment lengths. For their size, smaller bipeds have greater step lengths, limb excursion angles and duty factors than large bipeds because of their more crouched posture and greater effective limb length. The most notable differences in limb kinematics between birds and humans occur at the walk-run transition and are maintained as running speed increases. Change of gait is smooth and difficult to discern in birds, but distinct in humans, involving abrupt decreases in step length and duty factor (time of contact) and a corresponding increase in limb wing time. These appear to reflect a spring-like run that is stiff in humans (favoring elastic energy recovery) but more compliant in birds (increasing time of ground contact). Differences in balance of the body's center of mass in birds versus humans not only affects femoral orientation and motion, but also affects patterns of limb excursion with speed.