We have debated the effects of interspecific competition, especially in ecological time, on the distribution and abundance of dabbling ducks (Anas spp.), and the relative importance of bill morphology and body size in facilitating coexistence. Evidence from North American and Baltic coastal wetlands indicates that species with few lamellae (but longer bodies) tend to feed in shallow, vegetated microhabitats where invertebrate prey is large; species with dense lamellae (but shorter bodies) tend to feed offshore where prey is smaller. In northern Europe, however, the evidence is opposite, suggesting that differences in body length instead facilitate coexistence. Here, we present evidence that these views are compatible under a refined conceptual model. In both northern Europe and North America, microhabitat diversity within wetlands correlates with a-diversity (species per wetland). With increasing latitude, a-diversity decreases, coincident with changes in the shape of wetland basins from largely "saucer-" (prairie/steppe potholes) to "bowl-shaped" (nemoral/ boreal lakes and tarns). We propose that (1) there is less absolute microhabitat (water-depth/vegetation) gradient along which species can be accommodated in northern wetlands, owing to shoreline steepness, and (2) body length is more important in facilitating species co-existence among breeding dabbling ducks in "bowl-shaped" wetlands, whereas lamellar density is more important in "saucer-shaped" wetlands.