Questions 11-19

　　The origins of nest-building remain obscure, but current observation of nest-building

　　activities provide evidence of their evolution. Clues to this evolutionary process can be

　　found in the activities of play and in the behavior and movements of birds during mating,

　　Line such as incessant pulling at strips of vegetation or scraping of the soil. During the early

　　(5) days of the reproductive cycle, the birds seem only to play with the building materials. In

　　preparation for mating, they engage in activities that resemble nest-building, and continue

　　these activities throughout and even after the mating cycle. Effective attempts at construction

　　occur only after mating.

　　Although nest-building is an instinctive ability, there is considerable adaptability in

　　(10) both site selection and use of materials, especially with those species which build quite

　　elaborate constructions. Furthermore, some element of learning is often evident since

　　younger birds do not build as well as their practiced elders. Young ravens, for example,

　　first attempt to build with sticks of quite unsuitable size, while a jackdaw's first nest

　　includes virtually any movable object. The novelist John Steinbeck recorded the contents

　　(15) of a young osprey nest built in his garden, which included three shirts, a bath towel, and

　　one arrow.

　　Birds also display remarkable behavior in collecting building materials. Crows have

　　been seen to tear off stout green twigs, and sparrowhawks will dive purposefully onto a

　　branch until it snaps and then hang upside down to break it off. Golden eagles, over

　　(20) generations of work, construct enormous nests. One of these, examined after it had been

　　dislodged by high winds, weighed almost two tons and included foundation branches

　　almost two meters long. The carrying capacity of the eagles, however, is only relative to

　　their size ant1 most birds are able to carry an extra load of just over twenty percent of their

　　body weight.

　　11. The word "obscure" in line 1 is closest in meaning to

　　(A) interesting

　　(B) unclear

　　(C) imperfect

　　(D) complex

　　12. According to the passage, which of the following activities is characteristic of the early part of the reproductive cycle of birds?

　　(A) Selecting a mate

　　(B) Collecting nest-building materials

　　(C) Playing with nest-building materials

　　(D) Building a nest

(实习编辑：武勐勐)