Anatomy of the avian cecum.

The gross anatomy of the avian ceca is reviewed. In most birds, right and left ceca arise laterally or ventrolaterally at the junction of the small and large intestines. In a few species, the ceca open into the rectum ventrally or dorsally. In many herons and bitterns, only one cecum is present, and in the secretary bird there are two pairs of ceca. Ceca are absent in woodpeckers, hummingbirds, swifts, kingfishers, pigeons, mousebirds, cuckoos, and parrots. Ceca may be classified according to length into long, moderately or poorly developed, and vestigial types. In most birds, the ceca are simple tubular structures with minor variations in shape. However, in a few species, including the ostrich, rheas, kiwis, some tinamous, the red-throated loon, screamers, the satyr tragopan, the great bustard, and the pin-tailed sandgrouse, the ceca are sacculated or have diverticula. There is usually no correlation between the development of the ceca and systematic position. Except in grouse (Tetraonidae), in which the long ceca are related to the fibre content of the diet, the correlation between cecal development and diet is extremely limited. There is no relationship between the size of the ceca and the length and width of the rectum.