Beaks-bills for all occasions

Press, 25 July 1986, Page 18

Beaks-bills for all occasions

Birds can perform feats of dexterity with the beak which appear impossible for such a seemingly hard and insensible organ. In relation to feeding, there are beaks which are used for prising and striking, tearing, probing, sifting, spearing, . and grasping. The long, dagger-shaped bills of most herons and bitterns, and fish-eating birds in general, are especially shaped for seizing quick-moving prey. Some ducks, the mergansers, have saw-like teeth on the edges of their bills for grasping fish. Similar tooth-like structures or serrations for securing food are also seen on the edges of toucans’ bills. The outstanding

feature of toucans is that the bill is enormously enlarged — in some species it exceeds the body length and almost equals its bulk. The wading birds, for example the oystercatcher, godwit, and stilt, have elongated, slender bills, adapted for probing in mud or sand. Waders' bills, although characteristically slender, may be straight and pliable (the snipe), . deeply decurved (curlews), or in the case of avocets, recurved, an adaptation for feeding largely on the surface of shallow water.

The wrybilled plover, which is one of the few wader species in New Zealand which spends its entire life here, has a bill which is unique amongst birds — the whole of the distal half is curved to the right. I vividly recall one Canterbury Museum inquiry from an observant member of the public who reported seeing a white heron which was holding on to a 50 cent piece in the tip of its bill. After further discussion the bird was identified as a royal spoonbill. This species feeds on aquatic crustaceans, molluscs, insects, and small fish which it sifts from mud or water. Its bill is long, flattened in the horizontal plane and broadened to a thin spalulate tip ("D” in the accompanying illustration).

The New Zealand shoveler duck also has a wide, flattened, spoon-shaped bill; indeed many game-bird hunters refer to it as the "spoonie” or “spoonbill.” One group of birds, the skimmers, have bills which serve as a projecting scoop; they skim food from the surface of the water while in flight.

Other birds have bills which capture food in a manner similar to the sifting action of the baleen plates in the large toothless whales. In these birds, the bills are equipped with a series of elaborate plates (“K” on the illustration) or lamellae which sieve food from water. Lamellae are also present in the bills of swans, geese, ducks, flamingos, and in the prions, a group of small oceanic birds which are related to the petrels. Many perching birds or finches, for example the chaffinch, greenfinch, and house sparrow, have stout bills, conical in section and well adapted for crushing seeds. A species showing remarkable specialisation for cracking the stones of cherries 4*

and even olives is the hawfinch. The muscles of its jaw and skull are extremely powerful, and ridged bosses on the palate arid inside the lower jaw function as anvils. Transverse rasp-like ridges found on the inside of bills of certain parrots enable them to reduce the hardest fruit stones into fine particles. One of the most fascinating adaptations of a bird’s bill is seen in the crossbills; their mandibles are sharply pointed and widely crossed over to facilitate the rapid extraction of seeds from pine cones. Birds of prey have powerfully hooked and pointed bills, well shaped for tearing up their food. The hooked beaks of such scavengers as gulls and the giant petrel are also used for tearing flesh.

As a general rule mastication or the breaking up of food is not a function of the head region of birds. In many birds the food is broken up by a special part of the stomach, the ventriculus or gizzard. Unlike many other vertebrates the mouth of birds merely serves as a pathway for food to the pharynx and oesophagus. The lack of teeth and heavy masticating muscles in birds reduces their weight which, among a host of other structural adaptations, indirectly helps in flight

The ingenius ways in which birds seek and collect food must be an important factor in their success. Though many species keep strictly to one diet, others

are able to adapt to the food available at the time. The song thrush is an example; it can eat flesh (snails, earthworms, or caterpillars) or fruit Among the most specialised feeders are the humming-birds, eating nectar. The beak is long or short according to the length of the flower visited, and the tongue is pointed with a special tubular tip. Birds’ beaks serve many other purposes; as “hands,” for nestbuilding and preening, as weapons, and as, a structure which features prominently in social displays. Above all, they are used in feeding, when they show the greatest variety of special adaptation.

By

GEOFFREY TUNNICLIFFE