Gape

In bird anatomy, the gape is the interior of the open mouth of a bird and the gape flange is the region where the two mandibles join together, at the base of the beak. The width of the gape can be factor in the choice of food.

Gapes of juvenile altricial birds are often brightly colored, sometimes with contrasting spots or other patterns, and these are believed to be an indication of their health, fitness and competitive ability. Based on this, the parents decide how to distribute food among the chicks in the nest. Some species especially in the families Viduidae and Estrildidae have bright spots on the gape known as gape tubercles or gape papillae. These nodular spots are conspicuous even in low light. A study examining the nestling gapes of eight passerine species found that the gapes were conspicuous in the ultraviolet spectrum (visible to birds but not to humans). Parents may however not rely solely on the gape coloration, and other factors influencing their decision remain unknown.

Red gape color has been shown in several experiments to induce feeding. An experiment in manipulating brood size and immune system with Barn Swallow nestlings showed the vividness of the gape was positively correlated with T-cell–mediated immunocompetence, and that larger brood size and injection with an antigen led to a less vivid gape. Conversely, the red gape of the Common Cuckoo (Cuculus canorus) did not induce extra feeding in host parents. Some brood parasites such as the Hodgson's Hawk-cuckoo (C. fugax) have colored patches on the wing that mimic the gape color of the parasitized species.

When born, the chick's gape flanges are fleshy. As it grows into a fledgling, the gape flanges remain somewhat swollen and can thus be used to recognize that a particular bird is young. By the time it reaches adulthood, the gape flanges will no longer be visible.