RELATIONS BETWEEN ABUNDANCE, BODY-SIZE AND SPECIES NUMBER IN BRITISH BIRDS AND MAMMALS

British birds and mammals are compared in terms of their frequency dis tributions of abundance and body mass and in respect of the relation b etween abundance and body mass. Body masses of non-flying mammals are greater than those of resident birds which are, in turn, heavier than migrants; bats are lightest. The frequency distribution of masses are close to log-Normal for each of these groups, though their variances a nd skews differ. Differences in mean abundances (which are log-Normall y distributed) parallel those in body mass. In each group, abundance d eclines with body mass: the exponent of the relation is close to the v alue of -0.75 predicted by the 'energetic equivalence' rule though not significantly different from the value of -1.0 predicted by the 'biom ass equivalence' rule. At comparable masses, species of non-flying mam mals are more abundant than resident birds, migrant birds and bats by approximately 45, 300 and 200 times, respectively. The similarity betw een birds and bats in this regard may be no more than coincidental but it may be related to ecological similarities related to flight. The m etabolic rates of non-flying mammals may be generally lower than those of birds and bats but not sufficiently to account for their much grea ter abundances.