Birds use essentially the same genes to sing as humans do to speak. And flamingos are more closely related to pigeons than they are to pelicans.

These are some of the unusual discoveries that have emerged from the largest, most sophisticated mapping of the bird family tree, published in more than two dozen separate papers, eight of which are in the December 12 issue of the US journal Science.

Researchers in 20 nations spent four years on the project to sequence the entire genomes of 48 species of birds, including owls, hummingbirds, penguins and woodpeckers.

They also compared birds to three different species of crocodiles -- which are the reptiles most closely related to birds -- and found vastly different rates of evolution.

The birds were much faster at evolving new traits, while crocodiles -- which shared a common ancestor with birds and dinosaurs about 240 million years ago -- have barely changed at all.

Birds are "the one lineage of dinosaurs that made it through the mass extinction at the end of the so-called age of dinosaurs," some 65 million years ago, said co-author Ed Braun, associate professor from the University of Florida.

"Their closest living relative is actually crocodilian so you have again these very different organisms going back fairly deep in time."

A few kinds of birds are believed to have survived the catastrophic event that wiped out the dinosaurs, and from then on they evolved rapidly into the diverse array of some 10,000 species we see today.

- Traits emerge -

Birds lost their teeth some 116 million years ago, according to the research.

The urge to mate and be noticed by the opposite sex led to the rapid evolution of 15 pigmentation genes associated with plumage and feathers, said the findings.

Birds' ability to sing and mimic sounds is based on the same brain circuits seen in humans, though we arrived at these abilities by different evolutionary paths.

Meanwhile, chickens and ostriches are among the birds whose makeup most closely resembles that of their ancestors.

Co-author Erich Jarvis, an associate professor of neurobiology at Duke University School of Medicine, described discovering "a big surprise that it's actually the chicken that seems to have retained the most ancestral chromosomal organization compared to the other species.

"But it doesn't mean that other parts of the aspects of this genome are not as old. The ostrich could still be older," as it seems the ostrich genome is evolving more slowly than the chicken's, he told reporters.

Scientists were also surprised to find that flamingos, known for their long legs, elegant beaks and distinctive pink hues, are closely related to doves, pigeons and small freshwater diving birds known as grebes.

"And what we find is a real odd couple of birds... where we have doves and their allies and they're paired surprisingly enough with flamingos and grebes," said Braun.

"Flamingos and grebes are different enough looking -- although they're both water birds -- that you might be surprised to have them together but linking them with doves is especially unexpected."

To arrive at their conclusions, scientists used a variety of techniques including sampling bird DNA from frozen tissue in museums and using a process called "statistical binning" that helped them group and analyze more than 14,000 genes and build a family tree linking different bird species.

Overall, they found that size of birds' genomes are much smaller than most other vertebrates.

While researchers said they have come up with the most reliable avian tree of life to date based on all major types of birds, it is far from complete. More genomes need to be sequenced in the coming years to provide a fuller picture.