Food competition and niche separation between fish and the Red-necked Grebe Podiceps grisegena (Boddaert, 1783)

During the breeding season, the aquatic bird Red-necked Grebe Podiceps grisegena feeds on small aquatic animals, which are also an important food source for many fish species. Because grebes have to provide substantial amounts of invertebrates to their brood, competitive interactions with fish may be expected during the breeding season of grebes. Based on this hypothesis, the distribution of Red-necked Grebes, the abundances of macroinvertebrates, fish, and vegetation, as well as the water depth were determined in a shallow lake during two consecutive breeding seasons. The Red-necked Grebes only utilised the reed bed as breeding site, whereas the adjacent Lagoon, although potentially suitable as breeding site, was not used. Macroinvertebrate biomass was 5–10 times higher in the reed beds than in the Lagoon. The abundance of fish was low in the reed bed but considerably higher in the Lagoon. The results indicate habitat separation between breeding Red-necked Grebe and fish in Lake Tåkern, and that the separation is related to distribution of food resources birds and fish have in common.