Samples of Red-necked and Slavonian Grebes that had drowned in gill nets while foraging in winter were analysed for body size, nutrient reserve levels and food content. Measurements of Slavonian Grebes indicated that they may originate from the Icelandic and Norwegian breeding grounds, not from the Baltic area as is suggested in the literature. The mass of the bird after its nutrient reserves have been totally depleted, its ‘structural size’, is best predicted by different body measurements in each of the 2 species and in the Great Crested Grebe, even though they are closely related. Total fat mass was best estimated from fresh abdominal fat mass in both species. An average Red-necked Grebe in midwinter is estimated to have enough nutrient reserves to survive 11 days starvation. The diet of both species consisted exclusively of fish, mainly Smelt, followed by Perch and Ruffe. Both species took more, smaller Smelt than expected from the food supply that is present in the habitat. This suggests that swimming speeds of the fish may be important in determining whether grebes can catch them, so that food selection may be determined more by the agility of the bird than by its bill dimensions.