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

Controversy exists about whether (i) regular character (usually body s ize) ratios among coexisting species might result from interspecific c ompetition and (ii) migratory dabbling ducks (Anas sp.) might be limit ed by the abundance of invertebrate prey on wintering or breeding area s. We tested these ideas by comparing the extent to which coexistence of dabbling ducks on breeding and wintering grounds depended on differ ences in body size which, in turn, reflect differences in prey size. S pecies of dabbling ducks that differ in body size by less than a facto r of 1.2 co-occurred less frequently than expected by chance during th e breeding season when duck assemblages are characterized by low domin ance (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 spe cies co-occurrence in relation to differences in body size could not b e discerned from that which might occur by chance. This implies that c ompetition for invertebrate prey, at least, is unlikely during winter. These results contrast with data from assemblages of other sexually r eproducing vertebrate species, and with recent theories which predict that species' assortment patterns should be more regular than random i n assemblages characterized by high dominance. Our results are consist ent with the idea that communities of dabbling ducks are assembled, ex cept during winter, according to some limiting size similarity and are influenced more by interspecific interactions than by chance.