Abstract

The gizzards of eight species of birds were studied to investigate the relationship between diet, gizzard lumen volume and gizzard muscle movement and contractile force. The similar asymmetry of the gizzard muscles within and among species suggested that there is an ideal gizzard morphology, which generates a translational movement, for breaking down plant food. The narrowness of the gizzard lumen, even in gizzards with relatively large volumes, suggests the importance of this movement (which is most efficient when the lumen is narrow) to breaking down plant material. Heavier gizzards did not necessarily result in greater forces being applied to individual food items within the gizzard. However, they were capable of processing greater volumes of food. Discriminant analysis revealed that the greatest forces for breaking down food were to be found in grass-eating birds. It is suggested that breakdown of fibrous foods such as grass involves a trade-off, as a concession to weight reduction, between gizzard muscle mass, and hence the pressure generated within the gizzard, and the amount of food that can be processed at any one time.