Fligh speeds of bird have been clocked many times, but usually at ground speed, and it is asserted that migrating birds travel faster when migrating than they travel at other times. The speed of most small migrating birds seldom exceeds 30 miles per hour. Most shorebirds average between 40 and 50 miles per hour, whereas many ducks travel 50 to 60 miles per hour.　　　

　　At such speeds migrating birds may cover several hundreds of miles in a day or a night. It is routine for North American migrants to travel more than 500 miles nonstop across the Gulf of Mexico, where there are no midway islands. A turnstone, one of a cosmopolitan species of shorebird banded in Europe, covered 510 miles in 25 hours. Blue geese make the 1,700 miles from James Bay in Canada to coastal Louisiana in 60 hours.　　　

　　Most routine migration probably takes place within 3,000 feet of the Earth, although David Lack, computing the altitude of British nigh migrants with radar, found that some small land birds often travel at 5,000 feet and may sometimes go as high as 14,000 feet. Radar has recorded birds at altitudes of 20,000 feet, perhaps not too surprising since many species cross both the Andes and the Himalayas during migration.　　　

　　重点单词：

clock : n. 时钟,计时器,袜子上的花样;vt. 绣花样,记时,记录;vi. 记录时间

assert : vt. 主张,声明,断言

migrate : vi. 移动,移往,随季节而移居;vt. 使移居