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 requi