Constraints on dispersal and the evolution of the avifauna of the NorthernHemisphere

One could predict that the capacity for travelling great distances might pr edispose long-distance migratory birds to be good colonists and to exhibit wider geographic distributions than their non-migratory or short-distance m igratory relatives or non-volant mammals. This prediction is not supported by the data on avian biogeography. The distributions of species, genera and families of North American and Eurasian birds and mammals are indeed relat ed to migratory status, but long-distance migratory birds exhibit a great d eal of biogeographic regionalism. In particular, at all taxonomic levels th eir distributions tend to be confined to either the Eastern or Western Hemi sphere, suggesting that there has been little successful east-west dispersa l between North America and Eurasia. Compared to non-migratory birds, short -distance migratory birds and non-volant mammals, long-distance migrants ap pear to be subject to severe constraints on their physiology, behaviour and ecology, which have prevented colonization of distant regions.