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Non-native birds are replacing Hawaii鈥檚 endemic species, adapting to new environments at a blistering pace of evolution.

In the Hawaiian Islands, most native fruit-eating birds have gone extinct and been replaced by introduced counterparts. On the island of O鈥檃hu, seed dispersal through fruit consumption is now mostly carried out by non-native species.

Jason Gleditsch, of the University of Illinois at Urbana-Champaign, and his colleagues have been studying how these changes have affected the island. They have found that, despite the loss of native fruit-eaters, O鈥檃hu still has complex, functioning ecosystems 鈥� possibly thanks to rapid evolution of non-native birds to fill the niches vacated by the lost endemic species.

To see whether the introduced birds have been evolving on O鈥檃hu, Gleditsch and his colleague Jinelle Sperry compared four non-native songbird species on the island with museum specimens collected from their native range relatives. They measured the birds鈥� bills, tails, legs and wings 鈥� body parts that are all used for foraging.

They discovered that, since living on the island, these birds have undergone substantial physical changes of up to 13 per cent in some measurements over a remarkably brief period. Compared to native range specimens, two species had significantly shorter legs, and most species had larger, thicker bills, shorter wings, and longer tails.

The four species studied were introduced to O鈥檃hu only 50 to 90 years ago. 鈥淓volution is typically thought to occur over millennia,鈥� says Gleditsch. 鈥淲e鈥檙e talking about ten to twenty generations at most.鈥�

But statistical analysis suggests that the birds鈥� evolution is likely only partially shaped by adapting to life on the island. There鈥檚 evidence that 鈥渇ounder effects鈥� may also have been at work 鈥� an evolutionary phenomenon in which the genetic mix of a small starting population can prompt a species to evolve changes that aren鈥檛 necessarily useful or adaptive.

Gleditsch says it鈥檚 not yet clear if the birds are evolving to fill the niches of the newly-extinct native birds. It鈥檚 possible that the newer birds are mostly feeding on and dispersing the seeds of introduced plants, rather than adapting to native fruits. 鈥淚t may be a double-edged sword, where yes, they鈥檙e dispersing seeds, but they may not be dispersing the seeds we want them to,鈥� he says.

Natalie Wright, of Kenyon College in Ohio, says the findings provide a perspective on how quickly the original Hawaiian birds might have evolved after arriving on the newly-formed islands.

鈥淪tudying introduced species allows us to really understand how quickly change can occur in populations, and how often rapid change happens,鈥� says Wright.