Predictable evolution toward flightlessness in volant island birds

Area of Science:

• Evolutionary biology
• Island biogeography
• Comparative anatomy

Background:

• Birds are known for rapid evolution on islands, often developing unique traits.
• The "island rule" regarding body size changes is inconsistently applied to birds.
• Convergent evolution towards flightlessness is observed in island birds, involving muscle and limb modifications.

Purpose of the Study:

• To investigate if volant island bird populations exhibit subtle morphological changes converging towards a flightless form.
• To identify patterns in avian evolutionary changes in response to island environments and predation pressures.

Main Methods:

• Comparative analysis of flight muscle size and leg length in island versus continental bird species.
• Examination of 366 populations across the Caribbean and Pacific to correlate morphology with insularity and predator scarcity.
• Assessment of shifts in limb investment in relation to island size and predator presence.

Main Results:

• Island bird species possess smaller flight muscles compared to their continental counterparts.
• Reduced flight muscles and elongated legs are linked to increased insularity and decreased predator abundance.
• Morphological shifts favoring hindlimbs over forelimbs, mirroring flightless birds, were observed on smaller, predator-limited islands.

Conclusions:

• Island bird populations show a consistent evolutionary trajectory towards flightlessness, even while remaining volant.
• These predictable morphological changes can reduce flight capacity and limit dispersal-driven diversification.
• The observed patterns hold across diverse avian families and orders, indicating a widespread evolutionary trend.