What Happened to All the Birds in the Most Recently Colonized Regions of Earth?S

The numerous islands scattered throughout the Pacific Ocean comprise the last habitable region of Earth that humans colonized. Beginning about 3500 years ago, people settled the eastern islands, such as the Samoan islands, Fiji and the Marianas. By 700 years ago they made their way to the more remote locations of Hawai‛i and Rapa Nui (Easter Island). These islands once held hundreds of species of birds that disappeared when humans arrived. Now we have a clearer picture of what happened.

The Pacific islands experienced a massive die-off of bird species not long after human colonization, but just how many of the feathery creatures went extinct has remained a bit of a mystery to scientists. Now, a new analysis suggests that some 1000 nonpasserine landbird species (birds that don't perch) disappeared due to hunting and habitat loss between the time people first settled the Pacific islands and the arrival of Europeans.

These tropical landscapes were once dominated by numerous bird species, many of which became extinct in a relatively short time after humans arrived. Some conservative estimates say 800 species disappeared, while others place the number at over 2000. The main reason for this large extinction uncertainty is the spotty fossil record available, says Richard Duncan, an ecologist at the University of Canberra in Australia and lead author of the new study.

"Relatively few fossils have been collected from a lot of the islands that have been studied," Duncan told io9. Fossils will only preserve in certain types of sites — these "specialized habitats" are often difficult to find. "And a lot of the work on collecting raw data and fossils [in the Pacific] has been done by just a few people," he adds. The large differences between the islands, including area, topographic diversity and rainfall, further complicates getting an accurate rate of species loss, since these features would've affected how easily people could hunt and clear out forests.

So Duncan and his colleagues used the available data to refine the Pacific islands' extinction estimates. The team focused only on nonpasserine landbirds, which are better represented in the fossil record than sea birds and passerine birds. The large-bodied landbirds preserve as fossils easier than smaller birds and were likely the prime target of hunters, who left the remains in more accessible sites.

The team looked at the extinction question as a "mark-recapture" problem. Ecologists use this method to estimate the size of an animal population — it involves capturing and marking a portion of a population, then later collecting another group and counting the number of individuals that have been previously marked (recaptured). In this case, the landbird species recorded after the arrival of Europeans were the so-called marked species, which were available for recapture in the fossil record.

One way to think about it, Duncan explains, is to imagine you have an island with various species of birds. Say you count 10 species on the island, but find only five of them in the fossil record. The other five species should be there, you just haven't found them yet. This information can give you an idea about your ability to detect birds in the fossil record, and when combined with the number of extinct species discovered, it can help you to estimate the number of extinct species that remain missing and the associated rate of extinction. Though the researchers' model involves a bit more than this, it's basically "the idea of using the information we have on our ability to detect birds and extrapolating out to see how many species must be missing," he says.

The team performed their calculations for 41 Pacific islands. They found that the large, flightless birds were most likely to go extinct. And islands that are small and have little rainfall suffered the highest rates of extinction (about 90 percent). Overall, a whopping two-thirds of the nonpasserine landbird populations on the islands died off between colonization and European contact — this means that at least 983 bird species became extinct, an estimation that doesn't even include all of the seabird and passerine bird extinctions.

There are currently only 10,000 extant bird species today, so this event was quite significant in terms of the global bird diversity, Duncan notes. "To put it in context: This is probably the largest extinction event we've witnessed that was caused by humans," he says.

The researchers findings were published today in the journal PNAS.

Top image via Tim Blackburn.