49 According to the passage, which measure can effectively protect bird species?
(A) To monitor bird migration.
(B) To increase and preserve wetlands.
(C) To promote education in bird conservation.
(D) To develop medicine that can help birds live longer.

Many US bird populations are in decline, some of which are even on the brink（事情發生的）邊緣 of extinction滅絕；絕種.

Of the 800 species of birds in the United States, 67 are listed as endangered or threatened; another 184 are listed as “species of conservation（對動植物、自然地區、歷史建築等的）保育 concern” because they have small distribution分發，散發；分配, are facing high threats or have a declining population.

Hawaiian birds and ocean birds appear most at risk. More than one-third of all US bird species are in Hawaii. However, 71 Hawaiian species have gone extinct since A.D. 300 and about 10 species have not been seen in the past 40 years. The declines of US bird populations can be traced找到，查出，發現, 追查…的來源 to a variety of factors, including agriculture, climate change, unplanned urban development, and overfishing.

In the grasslands草原，草場, for example, intensified加強，增強 agricultural practices have hurt bird populations. Grassland cannot support many birds if it is overused or burned too frequently. Besides, public lands and parks are mowed刈，割 too frequently and the grass is too short to provide a habitat for birds.

While many bird species are in decline, some other species are thriving. Research shows that wetland濕地；沼澤地 bird species began to increase in the late 1970s, which reflects the importance of the artificial habitats to the survival of many bird populations.

The results prove that investment投資 in wetlands has paid off and that it is now necessary to invest similarly in other neglected habitats where birds are undergoing steepest不平緩的, （斜坡）陡的 declines.