Predictable evolutionary trends illuminate mechanisms that affect the diversity of traits and species on the tree of life. We show that when birds colonize islands, they undergo predictable changes in body shape. Small-island bird populations evolve smaller flight muscles and longer legs. These shifts in investment from wings to legs, although often subtle, are qualitatively similar to changes that have occurred in flightless bird lineages. Islands with fewer predator species were associated with more dramatic shifts toward flightlessness, implicating reduced predation pressure as the most likely cause of this trend. These predictable evolutionary changes likely exacerbate the vulnerability of flighted island birds to introduced predators and reduce the potential for small-island species to give rise to subsequent radiations.

Abstract

Birds are prolific colonists of islands, where they readily evolve distinct forms. Identifying predictable, directional patterns of evolutionary change in island birds, however, has proved challenging. The “island rule” predicts that island species evolve toward intermediate sizes, but its general applicability to birds is questionable. However, convergent evolution has clearly occurred in the island bird lineages that have undergone transitions to secondary flightlessness, a process involving drastic reduction of the flight muscles and enlargement of the hindlimbs. Here, we investigated whether volant island bird populations tend to change shape in a way that converges subtly on the flightless form. We found that island bird species have evolved smaller flight muscles than their continental relatives. Furthermore, in 366 populations of Caribbean and Pacific birds, smaller flight muscles and longer legs evolved in response to increasing insularity and, strikingly, the scarcity of avian and mammalian predators. On smaller islands with fewer predators, birds exhibited shifts in investment from forelimbs to hindlimbs that were qualitatively similar to anatomical rearrangements observed in flightless birds. These findings suggest that island bird populations tend to evolve on a trajectory toward flightlessness, even if most remain volant. This pattern was consistent across nine families and four orders that vary in lifestyle, foraging behavior, flight style, and body size. These predictable shifts in avian morphology may reduce the physical capacity for escape via flight and diminish the potential for small-island taxa to diversify via dispersal.