The comparative functional gizzard morphology of several species of birds

The gizzards of eight species of birds were studied to investigate the rela tionship 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 g enerates a translational movement, for breaking down plant food. The narrow ness 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 volum es of food. Discriminant analysis revealed that the greatest forces for bre aking down food were to be found in grass-eating birds. It is suggested tha t breakdown of fibrous foods such as grass involves a trade-off, as a conce ssion to weight reduction, between gizzard muscle mass, and hence the press ure generated within the gizzard, and the amount of food that can be proces sed at any one time.