They're starting their northern migration just about now, I'm not talking about birds, but rather these invertebrates. The swallows are now back in Capistrano, California, buzzards have returned to Ohio, ospreys have winged their way north to the Chesapeake Bay, and robins and other birds are either preparing to go north or are on their way. Birds aren't the only animals that migrate, though. Whales, eels, salmon and lots of other fish migrate to and from their breeding grounds. Herd animals, such as elephants and wildebeests migrate to where water and food is most plentiful.
Lemmings get an urge to move when local conditions become too crowded.
Whether you're big or small, it takes a lot of effort to move long distances. It's even tougher if you don't have a backbone. Most invertebrates move short distances compared to vertebrates. A journey from one part of a bay to another may constitute a migration for a clam. However, some insects migrate distances as long or nearly as long as do birds.