Re-writing Bird Evolution

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The largest ever study of bird genetics has not only shaken up but completely redrawn the avian evolutionary tree. The study challenges current classifications, alters our understanding of avian evolution, and provides a valuable resource for phylogenetic and comparative studies in birds.

Birds are among the most studied and loved animals, and much of what we know about animal biology -- from natural history to ecology, speciation, reproduction, etc. -- is based on birds. Nevertheless, the avian tree-of-life has remained controversial and elusive -- until now.

For more than five years, the Early Bird Assembling the Tree-of-Life Research Project, centered at The Field Museum, has been examining DNA from all major living groups of birds. Thus far, scientists have built and analyzed a dataset of more than 32 kilobases of nuclear DNA sequences from 19 different locations on the DNA of each of 169 bird species. The results of this massive research, which is equivalent to a small genome project, will be published in Science on June 27, 2008.

"Our study and the remarkable new understanding of the evolutionary relationships of birds that it affords was possible only because of the technological advances of the last few years that have enabled us to sample larger portions of genomes," said Shannon Hackett, one of three lead authors and associate curator of birds at The Field Museum. "Our study yielded robust results and illustrates the power of collecting genome-scale data to reconstruct difficult evolutionary trees."

The results of the study are so broad that the scientific names of dozens of birds will have to be changed, and biology textbooks and birdwatchers' field guides will have to be revised. For example, we now know that:

Birds adapted to the diverse environments several distinct times because many birds that now live on water (such as flamingos, tropicbirds and grebes) did not evolve from a different waterbird group, and many birds that now live on land (such as turacos, doves, sandgrouse and cuckoos) did not evolve from a different landbird group.

Similarly, distinctive lifestyles (such as nocturnal, raptorial and pelagic, i.e., living on the ocean or open seas) evolved several times. For example, contrary to conventional thinking, colorful, daytime hummingbirds evolved from drab nocturnal nightjars; falcons are not closely related to hawks and eagles; and tropicbirds (white, swift-flying ocean birds) are not closely related to pelicans and other waterbirds.

Shorebirds are not a basal evolutionary group, which refutes the widely held view that shorebirds gave rise to all modern birds.

"With this study, we learned two major things," said Sushma Reddy, another lead author and Bucksbaum Postdoctoral Fellow at The Field Museum. "First, appearances can be deceiving. 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."