They spent five years creating their tree, using millions of years worth of fossil data stretching back to Dinosaurs. They then mapped where on Earth and when in history birds' diversification took place. Their paper in Nature contains results of how 9,993 bird species currently alive globally made it to where they are today. Based on previous studies, the researchers expected to see bird speciation slowing down, but they instead found that birds' speciation rate is increasing, not declining.
"Perhaps birds are special," said Simon Fraser University biologist and study co-author Arne Mooers. "Maybe they're so good at getting around they can escape local competition from relatives and start anew elsewhere, producing bursts of new species at different times and in different parts of the globe."
The researchers also discovered that birds' speciation rate doesn't drop off the further they are from the equator. Since three quarters of all birds are found near the equator, it was expected that speciation there would be more common.
"We know the tropical biome has been shrinking during the last 15 million years," says Simon Fraser postdoctoral fellow Jeff Joy. "Perhaps, just as bushtits bunch together closely at night, bird species have clustered together in the tropics as their habitat shrunk."
Birds' rosy speciation history doesn't nullify the fact that they can't outfly extinction. Researchers estimate that birds have recently been proliferating at a rate of about one new bird species every 700 years but the recent human-caused extinction rate is about 300 times higher, they claim.
The main geographic differences in diversification rate are east west hemispheric rather than latitudinal, they note. Avian assemblages in Australia, Southeast Asia, Africa and Madagascar have low averages of diversification compared to the global mean. These regions also harbor substantially fewer than expected of the 25 per cent of species with the highest diversification rate.
They also found high diversification rates and large relative prevalence of top-diversification-rate species are found throughout higher latitudinal North America, parts of north Asia and southwest South America. These are the main breeding areas of several of the rapidly radiating bird species.