The first comprehensive map of hummingbirds' 22-million-year-old family tree -- reconstructed based on careful analysis of 284 of the world's 338 known species -- tells a story of rapid and ongoing diversification. The decade-long study reported in the Cell Press journalCurrent Biology on April 3 also helps to explain how today's hummingbirds came to live where they do.

Part of the secret to the birds' remarkable success lies in the formation of nine principal groups or clades, hummingbirds' unique relationship to flowering plants, and the birds' continued spread into new geographic areas, the researchers say.

"Hummingbirds have essentially been reinventing themselves throughout their 22-million-year history," says Jim McGuire of the University of California, Berkeley.

While all hummingbirds depend on flower nectar to fuel their high metabolisms and hovering flight, coordinated changes in flower and bill shape have helped to drive the formation of new species of both hummingbirds and plants. Remarkably, as many as 25 hummingbird species are able to coexist in some places.

"One of the really cool features of hummingbird evolution is that they all eat the same thing yet have diversified dramatically," McGuire says. "It really is a big surprise that hummingbirds have divided the nectarivore niche so extensively."