Meeting Abstract
Island birds have evolved flightlessness at least a thousand times. Each instance involved dramatic shape changes via the reallocation of mass from the forelimbs to the hindlimbs. The adaptive energy savings of this near elimination of costly flight muscles can only be realized by species that need not fly to forage and in the absence of terrestrial predators. The majority of island birds, however, retain flight. We hypothesize that these volant island species also reduce their investment in the flight apparatus, inducing a qualitatively similar but more subtle shape change. We examined shifts in the relative investment in forelimbs versus hindlimbs in 366 bird populations from 80 islands across the Pacific and Caribbean. We found that island-restricted volant species evolved smaller flight muscles than their continental relatives, with the degree of reduction greatest on islands of low species diversity. The decrease in flight muscle size was accompanied by an increase in leg length. This evolution along the trajectory toward flightlessness occurred in island bird populations from all nine families studied. We tested how reduced flight muscles and longer legs affect take-off performance in birds on the small, depauperate island of Tobago. We found that birds on Tobago had slower maximum velocity and maximum acceleration during take-off relative to conspecifics on the larger island of Trinidad. Initiation of wingbeats occurred later during take-off in populations on the island of Tobago. Lower predation pressures on small, species-poor islands likely permit the slower take-off velocities that result from island birds’ reduced flight muscles. These predictable morphological changes may explain why volant island birds are particularly vulnerable to introduced predators.
