As with other systems, the digestive system is modified for flight – a beak instead of teeth, a larger esophagus to swallow unchewed food, and a muscular gizzard for grinding. Various lengths and modifications of avian digestive tracts reflect bird’s habits. The jaws with their horny sheath together form the bill. Salivary glands are simple. The palate and membranous floor of the mouth are highly variable depending on the species’ diet. Pelicans have a very extended membranous floor of the mouth which they use to capture fish.
Tongues are also varied. Woodpeckers’ are long and sticky; hummingbirds, brushlike; honeyeaters’ tubular; etc. The only internal tongue musculature is found in parrots. Muscles to move the tongue in other birds is provided by the hyoid apparatus, a collective term for the five bones of the tongue.
To swallow food, a bird raises its head up or thrusts its head forward. To drink they raise their heads and let water run down the esophagus by gravity; only pigeons can actually suck fluid up.
In most species food is swallowed whole, but some crush seeds or berries or remove seed husks. Some seed-eating birds have special furrows in the bill to cut open the seed husk with a back and forth motion.
The food continues through the esophagus and into the crop, an expanded portion of the esophagus, particularly large in chickens, doves, and other grain eaters. The crop stores and may begin digestion of food and passes it to the stomach as the stomach empties. In several species of pigeons and doves, the lining of the crop is sloughed off and regurgitated to young as “pigeon milk“. Other species may store food for young in the crop and regurgitate partly digested (hawks, storks, pelicans, penguins, and some finches).
The stomach has two chambers, the proventriculus and gizzard; the former secretes gastric juice and the latter grinds. The gizzard has two sets of opposing muscles and may contain grit which may be helpful to help grind food but apparently isn’t essential. The gizzard may also serve as a filter for bones, feathers, fur, etc. which are regurgitated later (hawk and owl pellets, e.g.). Aquatic birds have a third chamber, filled with feathers, that filters out fish bones.
Birds digest food rapidly but that depends on the food and the bird. A Cedar waxwing can pass an ingested berry in 16 minutes!
The kidneys, as in mammals, maintain water balance, but are twice as large as mammals’ since birds have such active metabolism. There is no urinary bladder in birds, to save weight, except the Ostrich. The urine mixes with the fecal matter in the cloaca to form a white paste. The urine differs from that of mammals’ in its high concentration of uric acid rather than mammals’ urea. Mammalian urea is soluble in water and can be flushed out. But the uric acid of reptiles and birds is insoluble and thus can be excreted with less water in a paste form. This saves water as one gram of urea requires 60 ml of water to dispose of it while one gram of uric acid needs only three ml of water. Besides saving water and the weight of a urinary bladder, the reason that uric acid evolved is that birds and reptiles lay eggs. Urea and uric acid are both high in nitrogenous wastes- the breakdown products of protein. Concentrated nitrogenous wastes would be fatal to the embryo, so they are stored in the insoluble form of uric acid.
Some birds are restricted in their feeding habits (Everglades Kite, Penguins, Ivory-billed Woodpeckers, Osprey), while others are more ecumenical such as jays and crows. Most species are intermediate. Birds can be classified on their feeding habits as insectivores, granivores, nectarivores, piscivores, frugivores, etc.
Birds have a variety of beak specializations, but their diet may change seasonally. Both hummingbirds and sparrows will include insects in their diet when raising young – for more protein, and will eat more fatty foods to prepare for migration. Some birds, such as the Long-billed curlew or Woodcock, can open the tip of the bill while probing in the ground or mud or sand without opening the entire bill. In a number of birds such as orioles and starlings, the need to open their jaws (gape) to expose food inside a fruit or bud, in the ground, etc.
Birds’ bills are evolutionarily shaped by their environment, often food size or abundance.One example can be explained by the fact that insects are more abundant (both numbers and species) and get bigger in the tropics than in the more temperate regions. Thus with more to choose from, birds can specialize and vary over a greater range – especially on the bigger bill side. We often find that prey size and bill size are positively correlated, but not always.
The body size of the bird (not the bill) and prey size are sometimes correlated. See Lifjeld as example.