I was inspired to look up the yellow warbler's migration routes yesterday after Kat mentioned never having seen one. Here on the North Coast of Ohio, they seem most abundant during the spring migration, but lots of them stick around for the breeding season. We've seen them along riverbanks, where they frequently nest, when we've been out canoeing in midsummer.
As is the case with so many very small birds, one can only marvel at the feat of migration. Many yellow warblers cross the Gulf of Mexico twice each year. We need a boat or a plane; they require only two very tiny wings.