SPECIAL FEATURES OF BIRDS (3): The beak

The beak, or bill, is the bird’s main equipment for preening, feeding and attacking. The bill is adapted primarily for feeding and we know this by comparing the shape of the bill (bill morphology) with the actual feeding habits of the species.

For instance, Australia’s cockatoos have extremely strong and massively constructed beaks, and this makes them capable of cracking hard-shelled nuts and extracting banksia and casuarinas kernels. Honeyeaters and hummingbirds have slender beaks that are often curved, perfect for inserting into narrow flowers to extract nectar. In general, bills are specialised for grasping and manipulating food items.

While most adaptations of the bill for feeding have taken a long time to evolve, there are well documented instances when adaptations have taken place over a few generations in conditions of intense competition and/or changes in food availability. The most studied of these adaptations is the Galápagos finch, living in the very place where Darwin first conceived his theory of evolution. Here, scientists were able to see with their own eyes, from one season to the next, how fast evolution can work. One year, a drought caused most of the finches’ common food items to disappear, leaving behind only a very hard-shelled fruit. The finches with the strongest bills were able to crack the surface of the fruit and they survived.

The others perished. The survivors had offspring with the same strong beaks. Within one generation, a selection was made for a specific beak shape and strength.

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By developing wings, birds forfeited hands, quite a substantial handicap, one would think. Instead, the beak took over many of the functions of arms and hands. Birds use the beak to pick up, hold, throw and transport items such as twigs, stones and grasses. The beak is used for building nests, for wrestling with competitors and also for preening. Preening and scratching are particularly important for a bird’s survival. Preening involves the removal of debris and a waterproofing process, achieved by spreading over the feathers an oil derived from a preening gland located at the base of the tail feathers. Preening and scratching are also essential for defence against ectoparasites, such as lice, that live on the outside of the body (as compared to internal parasites) and feed on the body of the host. Without regular preening, birds would soon suffer from parasite overload, reducing their own chances of survival as well as their chances of reproducing successfully.

Bills and beaks may be well designed for feeding but not necessarily for preening. This is particularly noticeable in species with unwieldy beaks, such as the toucans and the hummingbirds. The beaks of toucans and hummingbirds are about the size of the bird’s body and are impossible to use for preening all parts of the body. In fact, the beak of the sword-billed hummingbird exceeds the length of its entire body. Despite this, the parasite load on these birds is no larger than in birds with beaks of a size and shape better suited to preening, because they have developed other antiparasite strategies to compensate for their beak limitations. To remove the ectoparasites, they rub against surfaces such as tree branches, sun themselves or bathe in the dust. They use their feet for scratching to remove parasites.

Even self-medication by ingestion or by keeping certain plants at the nest site may be used to inhibit ectoparasite build-up. Self-grooming (autogrooming) with the beak and feet may have its limitations if not all parts of the body can be reached but these too can be overcome. Birds that live in social groups have developed mutual grooming as an important social activity, just as in primates. This ‘allogrooming’ may well maintain a level of control over ectoparasites greater than can be achieved by self-grooming. (To be continue)

A Part of Introduction The Amazing Book of Gisela Kaplan & Lesley J. Rogers “BIRDS, THEIR HABITS AND SKILLS.

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1 Comment

  1. Maaf apabila pertanyaan saya keluar dari topik. Saya penasaran dengan bahasa Indonesia yang tepat untuk istilah ‘mutual feeding’, ‘mutual preening’, dan ‘bill-rubbing’. Setahu saya ketiga aktivitas itu biasa dilakukan burung dalam masa bercumbu.

    Terimakasih sebelumnya,

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