Migration, in biology, is the movement of animals to a place that offers better living conditions. Many birds, fishes, insects, and mammals regularly migrate to 1______ unfavorable changes in weather or food supply.
Migrations take place on land,in water, or in the air. Some animals migrate only short 2______. Other migrations cover thousands of miles or kilometers.
Migration enables many animals to 3______ favorable weather and abundant food supplies in areas with changing 4______. In some parts of the world, for example, food is plentiful in summer but becomes 5______ during the cold winter months. Many animals that live in these regions migrate to 6 climates in the fall. They return in the spring when the weather warms up.
Many migrations are 7______ reproduction. Numerous animals migrate to breeding areas so that their young have the best chance for 8______ Migratory birds breed in their summer homes, 9______ the food supply is most abundant.
Many animals begin their migrations after unfavorable environmental conditions 10______. But among other species, the factors that trigger (引发) migrations are more difficult to explain. Experiments show that changes in day length 11______ the migrations of many species of birds. In spring,the 12______ hours of daylight trigger the release of certain hormones in the bodies of the birds. Hormones are chemical substances that regulate many body functions.13______,the hormones prepare the birds for the northward trip.
Besides using environmental 14______, many seasonal migrators probably have an inborn “calendar” that tells them when to migrate. Some birds show seasonal migratory behavior even when kept 15 constant conditions in a laboratory. An inborn timing mechanism may trigger the migration of salmon and other animals that migrate at different stages of their life.
14
A.indexes
B.symbols
C.signs
D.clues
正确答案是D