With few predators, birds living on small islands don’t need large flight muscles for powering rapid escapes. According to researchers studying island populations in the Pacific and the Caribbean, these birds have smaller flight muscles and longer legs than their continental brethren. Though the shift is often subtle, island birds are evolving on a trajectory toward flightlessness – even if most retain their ability to fly. The findings are published in Proceedings of the National Academy of Sciences this week. 

Birds are prolific island colonizers, and once there, they evolve distinct forms readily. The so-called "island rule" predicts that animals evolve toward intermediate body sizes after colonizing islands, but this remains an inconsistent predictor of evolutionary trends for birds. In fact, the most striking evolutionary trend among island birds is the loss of flight – an irreversible transition has happened in more than a thousand lineages ranging from owls to pigeons (like the dodo) to rails (including the flightless weka pictured below). 

Becoming flightless means reallocating mass from the forelimbs to the hindlimbs, as well as the reduction of flight muscles, since they’re costly to keep. It typically involves two major requirements: the ability to forage without flying and an environment with very few (if any) predators. That explains why tens of thousands of island birds still fly. 

A trio led by Natalie Wright from the University of Montana, Missoula, wanted to see if flying (or volant) island birds tend to change their shape to converge on the flightless form. They examined island size, species richness, and the presence of predators (including raptors and mammals) for 366 bird populations on 80 islands across the Pacific and the Caribbean. They also weighed the two main flight muscles of 8,000 carcasses of both island and continental birds, and they measured the leg bones and sternal keels (for wing muscles attachment) of museum skeletal specimens spanning nine island families. 

Compared to their continental relatives, birds restricted to small islands have reduced flight muscles relative to body mass, and their bones show more investment in hindlimbs than forelimbs. This was the case even in birds that are unlikely to become flightless. This shift in investment from wings to legs (however subtle it may be) resemble changes that occurred in flightless birds. 

Furthermore, birds on islands with fewer predator species showed more dramatic shifts towards flightlessness, suggesting that freedom from predation is the most likely cause of this trend. However, that also means that island birds are especially vulnerable to introduced predators. 

^{Image in the text: Flightless weka (Gallirallus australis) in New Zealand pulling out a worm. Pi-Lens/shutterstock}