The relationship between abundance and body size is the subject of considerable debate in ecology1-15. Several data sets spanning a large range of body sizes show linear negative relationships between abundance and weight l-6 when these are measured on a logarithmic scale. But other studies of the abundances of species from single taxa, such as birds, which span a narrower range of body sizes reveal either little or no relationship, or a triangular relationship 9-15. Errors in estimating abundance might obscure relationships that do exist over a narrow range of body sizes. We describe here the relationship between body weight and abundance in British birds, whose population size estimates are unusually good. Abundance across all species declines with a -0.75 power of body weight, which conforms with the energetic equivalence 'rule'2,16. There is, however, a significant positive relationship between abundance and body weight within lower taxa. Those tribes that do not share recent common ancestry with other British birds are most likely to show a positive relationship across their constituent species. We thus show that phylogenetic relatedness might be an important indicator of the structure of the relationship between body size and abundance.
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