Rufous Hummingbirds (Selasphorus rufus) make the longest migration of any of the Trochilidae (Hummingbird Family) and, in proportion to size, one of the longest bird migrations.
It breeds as far north as Alaska and winters in Central America, a distance of ~2700 miles. This is is equivalent to 49,000,000 body lengths. This is the longest migration of any bird in terms of body length.