How Living on Islands Changes Bird Shape Around the World
Global study shows that island birds have rounder wings and longer legs, reflecting a shift toward terrestrial lifestyles in the absence of predators and rivals
Global patterns of change
Island life reshapes birds in predictable ways. A vast new study analysing almost 800 pairs of island and mainland bird species worldwide reveals that island birds tend to evolve longer legs and rounder wings—adaptations that make them more terrestrial and less reliant on flight. The work, led by Raquel Ponti and colleagues and published in *Global Ecology and Biogeography*, highlights the role of predator absence and reduced competition in shaping the evolution of island species.
Across 1,170 species, the researchers found that island birds consistently develop features linked to walking and ground foraging, while losing some of the aerodynamic efficiency needed for long-distance flight. Unlike the famous case of flightless rails, however, most of these birds retain their ability to fly—just less efficiently than their mainland counterparts.
Life without predators changes everything
On most oceanic islands, raptors and mammalian predators are few or absent. This relaxation of danger allows birds to spend more time on the ground, leading to longer tarsi (lower leg bones) that improve walking and foraging. The study found that this effect was most pronounced in passerines, a group that includes warblers, thrushes, and finches.
Conversely, islands that still hosted birds of prey showed a subtle reversal of this trend—shorter legs were more common where raptors persisted. These findings suggest that predator presence continues to shape the body proportions of island birds long after colonisation.
Competition—or lack of it—also drives evolution
With fewer closely related species competing for the same resources, island birds often adopt broader ecological niches. The researchers found that species living alongside few competitors evolved longer tarsi, supporting a more generalist lifestyle that allows them to forage in varied habitats.
“The absence of both predators and competitors promotes the evolution of more terrestrial, generalist birds,” the authors write. “Islands effectively relax the selective pressures that maintain flight efficiency.”
Wing shape reveals a loss of migration
The study also found that island birds had noticeably rounder wings than their mainland relatives, reflecting reduced flight performance. This effect was strongest when the mainland ancestors were migratory species, suggesting that sedentary behaviour evolves rapidly after island colonisation. Losing the need to migrate long distances removes evolutionary pressure for long, pointed wings suited to sustained flight.
Interestingly, the researchers found no significant change in wing length once body size was accounted for. This implies that the key evolutionary shift is not shorter wings, but rounder ones—better suited for manoeuvrability rather than endurance.
A widespread evolutionary pattern
These findings extend the classic “island rule”—the tendency for small species to grow larger and large species to shrink—to body shape. They show that insular birds not only change size but also shift their form to suit a more grounded existence. Ponti and her colleagues argue that this global pattern results from the combined effects of reduced predation, fewer competitors, and the abandonment of migration.
From the pigeons of Pacific archipelagos to the thrushes of Atlantic islands, island birds continue to illustrate how evolution follows predictable paths when life’s pressures change. The study’s vast scope and detailed trait measurements confirm that morphological evolution on islands is not just about size—it’s about how birds live, move, and survive when they have the ground to themselves.
November 2025
Read the full paper here
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