How birds evolve on islands

New research from the University of Montana has discovered a predictable trend in the evolution of bird shape. The findings, published in Proceedings of the National Academy of Sciences, showed the birds on islands have evolved towards flightlessness.

Lead researcher Natalie Wright explained: "The search for general trends in evolution of animal shape, size and colour often comes up empty. Evolution tends to be unpredictable, leading toward different forms in different places. So it was gratifying to discover this trend among island birds."

It is widely known that particular types of birds tend to become flightless after colonising predator-free islands, though it tends to evolve only from certain types of ancestors — usually birds such as rails, which spend most of their time on the ground. Wright wanted to study whether any evolutionary trends applied to all birds, including those that still need to fly in order to find food. The team therefore focused on how flying birds evolved on islands and whether they exhibit similar changes to those seen in flightless island species.

Wright and her team found that flying birds evolved smaller flight muscles and longer legs on islands, just as occurred in flightless birds, but to a lesser extent. On islands with fewer predators, this evolutionary trend toward flightlessness was more pronounced. This trend held for bird species that rely upon flight for finding food, including those representing families of birds that have never had a species member completely lose flight — such as hummingbirds, kingfishers and flycatchers.

Wright commented: "I would be shocked if a hummingbird or tanager or fruit-dove ever lost the ability to fly entirely — it's central to how they make a living. But these birds are still reducing the size of their flight muscles when relief from predators allows them to do so."

Found throughout the Caribbean, Bananaquit has smaller flight muscles and longer legs on islands with fewer predators (Photo: P Garrity)

Wright and her colleagues examined how well various island characteristics — including island area, overall bird species richness, raptor species richness and the presence of native mammalian predators — predicted changes in body shape in birds. They found that a combination of richness in raptor species and mammalian predators best explained variation in flight-muscle size and leg length, suggesting that predation rather than something else inherent to being restricted to islands has driven the pattern.

Wright added: "Without predators selecting for the maintenance of large, powerful flight muscles, birds can save energy by reducing these muscles. Instead, they develop longer legs as they rely more heavily on leg-powered locomotion. These longer legs may act as a longer lever as the birds jump during takeoff, making up for their smaller muscles."

These findings also shed light upon a mechanism for a long-known trend: island birds, even those that fly, are extremely vulnerable to introduced predators. Many scientists, including Darwin, have remarked upon island birds' seeming lack of fear of predators. This 'island tameness' has been suggested as a cause of island birds' susceptibility to predators. The present findings suggest another contributing factor: small flight muscles that reduce the speed of escape.

Islands are often hotbeds of evolution because their isolation facilitates genetic divergence from related populations, but systematic evolution of smaller flight muscles may also make it less likely that island populations will colonise new areas from those islands.