Biological diversity is a product of evolution and any attempt to understand it must recognize the relationships between morphology and ecology. Ecological morphology is broadly concerned with making connections between how organisms are constructed and the ecological and evolutionary consequences of that design. This implies that ecomorphology implicitly is concerned with fitness and hence adaptation. One of the central paradigms in ecomorphology focuses on the role of organismal performance as a crucial link between the organism's phenotype and its ecology. Performance will constrain the range of environmental resources that individuals can exploit, and influences energy intake which affects fitness components such as mating success, clutch size, and brood survival. This project deals with the study of the interelationships among morphology, performance (flight) and fitness in Delichon urbica (House Martin), Hirundo rustica (Barn Swallow) and Riparia riparia (Sand Martin), three passerine species, comparing northern and southern populations. The phylogenetic distances among the species will allow a test of the relationship between sexual selection and functional morphology at an evolutionary time scale. From a biological perspective, the species differ in foraging behaviour, morphology, and reproductive biology allowing tests of how evolutionary changes in morphology affects aerial performance and reproduction. The biogeographical approach will enhance evolutionary explanations of biological diversity that also have implications for biological conservation of species.