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 Newfoundlands 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 -turningmentioned 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)Newfoundlands conifers have evolved small cones
(D)Several subspecies of crossbills have evolved from two species
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