Ecomorphology and coexistence in dabbling ducks: the role of lamellar density and body length in winter

Ecomorphological differences allow species to coexist within communities, as animals with different morphological characters are likely to use distinct niches, hence reducing interspecific competition. Ecomorphological studies have highlighted the role of e.g. bill size and shape in passerines (Newton 1967), body size in amphibians, reptiles, insects and rodents (Barbault 1992), canine diameter in carnivores (Pimm and Gittleman 1990), neck height and/or incisor arcade structure in herbivorous mammals (Gordon and Illius 1988, Du Toit 1990). Because competition for food has long been a keystone of community ecology, it is not surprising that ecomorphological studies have often focussed on characters related to animal’s foraging habits. Anatidae, especially dabbling ducks (Anas spp.), have often been presented as a classic example of how subtle morphological differences may allow congeneric species to coexist (Lack 1971). All Anas species are morphologically similar, i.e. broad and elongated body with welldeveloped neck, and flat bill lined internally with lamellae. However, body length and lamellar density differ between species and these differences have been considered as the means by which duck species can coexist: because dabbling ducks almost never dive to forage, but may have to up-end to reach their food at the bottom of waterbodies, differences in body length allow species to segregate along a foraging depth gradient (Thomas 1982, Poysa 1983, Poysa et al. 1994, Green 1998). Ducks use bill lamellae to filter the water or the mud and retain food particles, and differences in lamellar density may allow ducks to segregate along a prey size gradient (Thomas 1982, Nudds and Bowlby 1984, Nudds et al. 1994). Body length versus lamellar density: controversy and agreement at the Holarctic scale