Migration,in biology, is the movement of animals to a place that offers better living conditions. Many birds, fishes, insects, and mammals regularly migrate to16unfavorable changes in weather or food supply. 

Migrations take place on land, in water, or in the air. Some animals migrate only short17Other migrations cover thousands of miles or kilometers. 

Migration enables many animals to18favorable weather and abundant food supplies in areas with changing19In some parts of the world, for example, food is plentiful in summer but becomes20during the cold winter months. Many animals that live in these regions migrate to21climates in the fall. They return in the spring when the weather warms up. 

Many migrations are22reproduction. Numerous animals migrate to breeding areas so that their young have the best chance for23Migratory birds breed in their summer homes,24the food supply is most abundant. 

Many animals begin their migrations after unfavorable environmental conditions25.But among other species,the factors that trigger(引发) migrations are more difficult to explain. Experiments show that changes in day length26the migrations of many species of birds. In spring,the27hours of daylight trigger the release of certain hormones in the bodies of the birds. Hormones are chemical substances that regulate many body functions.28，the hormones prepare the birds for the northward trip. Besides using environmental29, many seasonal migrators probably have an inborn u calendar that tells them when to migrate. Some birds show seasonal migratory behavior. even when kept30constant 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. 

16.（）