The riddle of the sands: why are shorebird densities so high in Southern estuaries?

Population data for migratory waders (Aves: Charadrii) during the middle of the nonbreeding season at 31 coastal wetlands in the western Palearctic and Ethiopian regions were analyzed to examine the relationships between bird density and energy intake, intertidal foraging area, and latitude. Wader density is closely linked to population energy intake at individual sites during the nonbreeding season and increases from northern to southern latitudes independently of wetland size. A simple mathematical model linking the variations in the seasonalities of invertebrate production and bird predation is used to provide a mechanistic explanation of the observed variation in bird density with latitude. Results are discussed in the light of current competition- and survival-based models of wader distribution, and, contrary to predictions of some models, we conclude that wader populations track the carrying capacities of coastal wetlands across a wide latitudinal range.