The island syndrome in birds

Abstract The island syndrome is a widespread biological phenomenon that describes a suite of morphological, behavioural, demographic and life‐history changes associated with island dwelling. These similar evolutionary responses among disparate groups of animals and plants represent a remarkable case of convergent evolution. Among animals, birds are a highly suitable group to study the island syndrome; they are a comparatively data‐rich taxon, are frequent island colonisers, and sometimes display extreme adaptations such as the loss of flight. However, the avian island syndrome literature is fragmented, and multiple components are rarely considered together even though many are inextricably linked. We reviewed multi‐species comparative studies, single‐species or population‐level studies and anecdotal accounts, to summarise and assess the support for individual components of the island syndrome for birds, and to identify suites of traits that should be considered together. The weight of evidence for island syndrome patterns in morphology is substantial, but is more partial or even anecdotal for various aspects of behaviour, life history and physiology. Full validation of the island syndrome in birds will require the less‐studied components to be treated in a comparative framework, and for covarying components to be examined in an integrated way. An improved description of the scope of the syndrome will pave the way to understanding its drivers.