Biology[img]2012q1/ct_etoefm_etoeflistz_0555_20121[/img] [br] What is the main i

Questions 23 through 28．Listen to part of a talk in a biology class.
The bird’s bill is composed of a horny sheath over a bony core. The entire structure has evolved in numerous ways to the specialized needs of its owner. The bird’s bill is highly modified for a variety of activities—such as cutting and crushing seeds,  probing tree crevices for insects,  drinking nectar from flowers,  catching fish,  and so on. There’s a large variety in the diets of birds throughout the world. Different birds have evolved different bill types to assist in their feeding. What I mean is,  the shape of the bird’s bill is specially adapted to specific foods.
A large number of birds are primarily seed eaters. These birds typically have stout, cone-shaped bills and strong jaw muscles for crushing seeds. The edible kernel of a seed is protected by a husk, or shell, which must be removed before the seed can be digested. The smaller finches, sparrows, and grosbeaks use their heavy conical bills to process seeds by removing the outer covering before swallowing them. Larger birds, such as pigeons and pheasants, swallow seeds whole and rely on grit in the gizzard to remove the husks and crush the seeds.
A wide variety of birds are nectar drinkers. These birds prefer the sweet liquid nectar of flowering plants. The co- evolution of birds and flowers has led to specialization, in which the bird’s bill is adapted for the collection of nectar. The bill evolved so that it could reach the nectar at the bottom of a flower’s long tubular corolla. Birds who drink nectar have long, narrow bills and tongues with finely divided tips that form a brush. The tongues are very long,  and the edges roll inwards to make a narrow scoop, so the bird can sort of lap up nectar, taking several licks every second. The tongue holds the nectar by capillary action—a physical force that causes fluids to rise in narrow tubes. The tongue moves in and out rapidly, carrying nectar up tiny grooves in the tongue,  all the way to the mouth.
Hummingbirds can stick their long tongues out far beyond the tips of their bills. This enables them to reach the nectar at the base of flowers. But along with drinking nectar, hummingbirds will also grab any insect they meet. As they flit from flower to flower, they consume the flower’s nectar as well as insects and spiders that provide them with protein.
Birds that eat insects collect their food in a number of ways. The warblers have short, slender, tweezer-like bills, that they sort of use like tweezers to pick small insects off leaves and twigs. Plovers pluck insects from the soil with their short bills. Starlings have longer bills, which they push into the earth and force open to make a hole, and then look down the hole for insects.
The insect eater’s tongue—like the bill—is specialized to the bird’s diet. The tongue of some woodpeckers is long, sticky, and barbed, which makes it easier for the bird to catch its insect prey. Several species of woodpeckers use their bills as wedges to pry the bark off trees to reach the insects underneath and then lap them up with their long, sticky tongues.