BODY-SIZE AND SEASONAL COEXISTENCE OF NORTH-AMERICAN DABBLING DUCKS

Controversy exists about whether (i) regular character (usually body size) ratios among coexisting species might result from interspecific competition and (ii) migratory dabbling ducks (Anas sp.) might be limited by the abundance of invertebrate prey on wintering or breeding areas. We tested these ideas by comparing the extent to which coexistence of dabbling ducks on breeding and wintering grounds depended on differences in body size which, in turn, reflect differences in prey size. Species of dabbling ducks that differ in body size by less than a factor of 1.2 co-occurred less frequently than expected by chance during the breeding season when duck assemblages are characterized by low dominance (i.e., the fractional abundances of species are roughly similar). In winter, when duck assemblages are characterized by high dominance (i.e., a few abundant species and many rarer ones), the pattern of species co-occurrence in relation to differences in body size could not be discerned from that which might occur by chance. This implies that competition for invertebrate prey, at least, is unlikely during winter. These results contrast with data from assemblages of other sexually reproducing vertebrate species, and with recent theories which predict that species' assortment patterns should be more regular than random in assemblages characterized by high dominance. Our results are consistent with the idea that communities of dabbling ducks are assembled, except during winter, according to some limiting size similarity and are influenced more by interspecific interactions than by chance.