The crural musculature in 19 species of birds was examined. New data on the mechanical properties of avian tendons showed a mean linear elastic modulus of about 1.3 GPa and failure stress of 74MPa. Physiological cross-sectional areas were calculated for 12 muscles in each species, together with the cross-sectional areas of their attached tendons. Investigation into potential various sources of error demonstrate this is a robust method for studying muscle-tendon units. The mode for calculated peak tendon stresses was 17.5 MPa. The results were compared to those obtained from mammal limbs by Ker, Alexander & Bennett (1988). It appears that the muscle-tendon units in birds are not as strongly optimized for the minimization of mass as those in the limbs of mammals. This may reflect a compromise between optimization for minimum mass and other functional requirements.