According to some scientists, migratory (迁徙的) birds should
be able to withstand (抵抗) the winter. A bird’s feathery coat is a good
protection against the cold. Because a bird is warm-blooded, its body
temperature always remains unchanging, even if the temperature of its
surroundings (周围环境) changes. The factors (因素) that bring about
migratory behavior (行为) in birds are difficult to explain. It seems that this
kind of behavior is not be learned but inborn. For example, many northern birds
leave their summer homes while the weather is still warm and there is still
enough food to eat. Bird migrations are probably regulated (调整)
by the glandular system (腺性分泌系统). Scientists think that the changing length of
the day is the factor that brings about migratory behavior. In an experiment,
migratory birds were kept in artificially (人工地) lighted rooms. It was found that
if periods of darkness were lengthened proportionately (按比例地), the glands of the
birds became active. These glands secrete hormones (荷尔蒙), which are
chemicals that control lots of body functions (机能). Shorter periods of daylight
seem to change the hormone balance of birds, so that they retain more fat. This
stored fat is the fuel that provides the energy for a long flight. The same
experiment showed that the birds became more excited as the artificial night was
lengthened. It is probably no coincidence (巧合) that most birds begin their
migratory flights during the night. The underlined word "secrete" (line 5, para.3) is closest in meaning to
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