A five-year long project to assemble the family tree of birds has been completed, throwing up a few surprises and challenging current classifications.

The Early Bird project, part of the US National Science Foundation's 'Assembling the Tree-of-Life' research program, published some of its findings in today's issue of the journal Science.

Some of the findings include: falcons are not closely related to hawks and eagles; the colourful hummingbirds evolved from drab nightjars; and tropicbirds - white, swift-flying ocean birds - are not closely related to pelicans and other water birds.

In fact, the study found that birds have evolved several times. Some birds that now live on water, such as flamingos and grebes, did not evolve from a different water bird group, and many land birds, such as doves and cuckoos, did not evolve from a different land bird group.

One of the main findings is that shore birds are not a basal evolutionary group, refuting the widely held view that shore birds gave rise to all modern birds.

The study also suggests that ecological niches such as a nocturnal lifestyle or raptorial hunting may also have evolved multiple times.

One of the study's lead authors, Sushma Reddy from Chicago's Field Museum of Natural History, said the team had learned two things from the project.

"First, appearances can be deceiving," she said. "Birds that look or act similar are not necessarily related. Second, much of bird classification and conventional wisdom on the evolutionary relationships of birds is wrong."

The researchers examined 32 kilobases of DNA sequences from 19 independent loci for 169 species, representing all of the major extant groups.