Links between the abundance and distribution of birds

The relationship between overall abundance, local abundance and distribution is examined for several taxonomic groups of birds in Britain Among closely-related species, all three measures were correlated those species with the highest overall numbers also had the highest local numbers and the widest distributions Within each group, the relationship between overall abundance and distribution was extremely tight (with all the species points close to the trend line) in plots of log distribution (numbers of 10 km squares occupied) against log numbers The pattern held in summer and winter, and in both resident and migratory species Moreover, species which, over a period of years, underwent marked change in status showed parallel changes in both breeding numbers and distribution, and followed the pattern found from comparisons between related species Conversely, species which changed little in numbers over a period of years also changed little in extent of distribution
An explanation of the findings is proposed, based on density-dependent growth of local numbers and dispersal The relevance of the findings to the limitation of geographical ranges is discussed The distributional extent of many species seems to depend primarily on their overall numbers, rather than vice versa