Are Island Birds Giants? Testing Foster’s Rule Across All Passerines

Species on isolated island systems often experience different environmental conditions compared to their counterparts on the mainland. These conditions can provide unique challenges and opportunities and the isolation can even lead to insular dwarfism in large animal species, and insular gigantism in small species – a process known as Foster’s rule. The goal of this study was to perform a large-scale analysis to determine if members of the order Passeriformes (perching birds) follow Foster’s rule, with island species exhibiting comparatively larger body sizes. Being one of the most diverse groups amongst vertebrates, passerines often also have high rates of dispersal and can be found on many unique island habitats. I analyzed the masses of over 5,000 extant species endemic to either islands or continental regions around the world. I then conducted phylogenetically-controlled analyses to account for shared ancestry and demonstrate clear support for Foster’s rule in passerines