Species on islands are widely believed to undergo similar evolutionary changes as an adaptation to the island environment, a process referred to as the ‘island syndrome’. These changes affect morphological, reproductive, physiological and behavioural traits such as sexual selection. Evolutionary change on islands is believed to relate to genetic (e.g. bottleneck) and ecological factors (e.g. release from natural enemies such as parasites and predators). However, very few broad-scale tests of these patterns have been conducted and few detailed studies examined the underlying processes. Additionally, previous studies were mostly based on data obtained from the literature, which are typically variable in quality. This is surprising given the central role of islands in ecological and evolutionary theory. Whether an ‘island syndrome’ does exist or not can only be established by demonstrating that a set of traits has arisen independently several times in unrelated taxa and geographically distinct areas. We propose to examine whether there are consistent patterns of adaptation in morphological, reproductive and sexually selected traits of island birds worldwide and to investigate some of the underlying causes. To this end, we will first collect standardised data from museum specimens for islands and nearby mainland areas from around the world consisting of morphological, egg and plumage colour measures. Subsequently, we will investigate in detail some of the underlying processes and consequences of evolutionary change on islands in the Gulf of Guinea using weaver (Aves, Ploceidae) species, for which phylogeographic data is available, on the island of São Tomé and nearby mainland Gabon. Specifically, we will address the interaction between decreased parasite pressure, immunity, intensity of sexual selection and reproductive investment to elucidate some of the constraints and trade-offs that lead to changes on islands.