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How much does habitat isolation drive forest bird morphology?

Rapid environmental change caused by humans has become a major concern for wildlife conservationists. But phenotypic and evolutionary responses of species to such change may often be swift enough to prevent their collapse. Several North American bird species living in boreal forests now have more pointed wings (a proxy for sustained ight efficiency), than they had a century ago. This remarkable pattern has been hypothesized as resulting from selective pressures favoring colonization of isolated habitat. Additionally, aerodynamics predict that heavier birds can achieve faster ight, a further advantage for exploring isolated habitats. We tested whether birds establishing territories in isolated areas have more pointed wings and are heavier than con-specifics found in more densely populated areas. Wing shapes of wild-caught adults from 21 passerine bird species did not generally support this prediction. However individuals with large body mass relative to their species were found more frequently in isolated habitats. Our results offer partial support for the isolation hypothesis at the landscape scale. We encourage further work at coarser, regional, scales to further examine whether wing shape and body mass evolutionarily respond to habitat isolation.