Migratory behavior and flight metabolism are influenced by many neuroendocrine factors. In fish engaged in migration from fresh water to the sea, prolactin and/or thyroid hormones often play key roles in migration and salinity preference. Prolactin induces migration to the water and the changes of second metamorphosis in a number of amphibians, and thyroid hormone may stimulate movement away from water. In birds there is evidence that prolactin, cortical steroids, thyroid hormones, gonadotropins and gonadal steroids can all influence migration; considerable interspecies variation exists. Juvenile hormone stimulates oogenesis and migratory behavior in several insects, but has no effect or causes flight muscle degeneration in others. It may serve to coordinate oogenesis, adult diapause and migration, particularly in colonizing species. Other neuroendocrine products have been implicated in control of migratory behavior or flight metabolism of insects including ecdysone, adipokinetic hormones and octopamine.