The differences in the body sizes observed in island birds versus their closest mainland relatives have puzzled biologists for decades. First noted in other vertebrate groups by Dr. Bristol Foster, the general trend is usually summarized as small mainland species evolving larger bodies on islands,while typically large mainland species shift towards dwarfism. With many examples in both living and extinct fauna, the overall phenomenon became known as Foster’s rule. Herein, we examine Foster’s rule as it applies to class Aves (the group that contains all modern birds). We analyzed the body mass (n=9,316), body length (n=7,260) and wingspan (n=708) of avian species from around the world. To account for phylogeny, we employed the use of multiple independent phylogenetic generalized least squares analyses (PGLS). We first analyzed class Aves as a whole and then looked at each individual order separately. We found support for Foster’s rule in class Aves overall, island species heavier and longer than mainland species. Wingspan did not vary between island and mainland species for the class as a whole. Looking at each order, we found that body size varied between islands and mainland in Anseriformes, Accipitriformes, Charadriiformes, Galliformes, Piciformes, Pelecaniformes, and Strigiformes. Whereas body mass in Galliformes and Piciformes increased on islands, Anseriformes decreased. Similarly, body length in Piciformes and Pelecaniformes increased, while Anseriformes and Strigiformes length decreased. Wingspan increased in both Accipitriformes and Charadriiformes. Although we found support for Foster’s rule, the direction of effects varied by order, and by body size metric, indicating that the biogeographical pattern is not generalized across class Aves as has been previously suggested, and underscores the idea that body size evolution may be driven by a combination of stochastic and deterministic forces specific to bird orders.
