Question 18-31

In the world of birds,bill design is a prime example of evolutionary fine-tuning.Shorebirds such as oystercatchers use their bills to pry open the tightly sealed shells of their prey,hummingbirds have stiletto-like bills to probe the deepest nectar-bearing flowers,and kiwis smell out earthworms thanks to nostrils located at the tip of their beaks.But few birds are more intimately tied to their source of sustenance than are crossbills.Two species of these finches,named for the way the upper and lower parts of their bills cross,rather than meet in the middle,reside in the evergreen forests of North America and feed on the seeds held within the cones of coniferous trees.

The efficiency of the bill is evident when a crossbill locates a cone.Using a lateral motion of its lower mandible,the bird separates two overlapping scales on the cone and exposes the seed.The crossed mandibles enable the bird to exert a powerful biting force at the bill tips,which is critical for maneuvering them between the scales and spreading the scales apart.Next,the crossbill snakes its long tongue into the gap and draws out the seed.Using the combined action of the bill and tongue,the bird cracks open and discards the woody seed covering action and swallows the nutritious inner kernel.This whole process takes but a few seconds and is repeated hundreds of times a day.

The bills of different crossbill species and subspecies vary -some are stout and deep,others more slander and shallow.As a rule,large-billed crossbills are better at securing seeds from large cones,while small-billed crossbills are more deft at removing the seeds from small,thin-scaled cones.Moreover,the degree to which cones are naturally slightly open or tightly closed helps determine which bill design is the best.

One anomaly is the subspecies of red crossbill known as the Newfoundland crossbill.This bird has a large,robust bill,yet most of Newfoundland's conifers have small cones,the same kind of cones that the slender-billed white-wings rely on.

18.What does the passage mainly discuss?

(A)The importance of conifers in evergreen forests

(B)The efficiency of the bill of the crossbill

(C)The variety of food available in a forest

(D)The different techniques birds use to obtain food

19.Which of the following statements best represents the type of "evolutionary fine -turning"mentioned in line1?

(A)Different shapes of bills have evolved depending on the available food supply

(B)White -wing crossbills have evolved from red crossbills

(C)Newfoundland's conifers have evolved small cones

(D)Several subspecies of crossbills have evolved from two species

20.Why does the author mention oystercatchers,hummingbirds,and kiwis in lines 2-4?

(A)They are examples of birds that live in the forest

(B)Their beaks are similar to the beak of the crossbill

(C)They illustrate the relationship between bill design and food supply

(D)They are closely related to the crossbill

21.Crossbills are a type of

(A)shorebird

(B)hummingbird

(C)kiwi

(D)finch

22.Which of the following most closely resembles the bird described in lines 6-8?

(A)（图）

(B)（图）

(C)（图）

(D)（图）