I thought that was odd, too. But apparently the man who coined this term (Alexander von Middendorff) did so in the context of a bird migration study, so that probably explains it. You probably also know that the same man coined the term radula in a mollusk study. :-)
On a related note, "migratory eels" has a nice ring to it.
Thought you'd like it, reesetee. Seems strange that the definition is restricted to birds though. Today's newspaper (Philadelphia Inquirer) had a map showing when monarch butterflies reach various areas as they migrate to their wintering grounds in Mexico. And what about migratory wildebeest and eels? Or phenology more broadly?