The paper is based on measurements of 2925 grebes and extensive field studies on ecological segregation and resource exploitation of the grebes of northern Europe, Colombia, Peru, Patagonia and Australia.

In all examined cases where two closely related grebe species overlap locally, one or both showed indications of divergent bill morphology. In two cases there are indications of evolutionary changes during the present century. In the case of the grebes of the Junin and Titicaca basins and for one European species, it was found that the morphological change reduced the interspecific food overlap.

There is a statistical tendency for evolution of a quite specific bill (21–26 mm long, 88-9-8 mm high, 7-6-8-5 mm wide) in isolated areas with only one grebe species. A close examination of one of these populations suggests that this bill type is an ‘all-purpose’ one which permits opportunistic fish-eating without loss of the ability to feed efficiently on tiny arthropods. In the case of two or more sympatric grebes, there appears, on a worldwide scale, to be a compulsory radiation away from the ‘all-purpose’ bill type.

The data suggest the existence of co-evolution in grebe faunas. Birds living under poor feeding conditions have to exploit all potential foods available, while specialization on optimal foods takes place where it is easy to find. Small individual differences in the trophic organs thus influence the diet only in relatively prolific habitats. Character displacement is therefore most likely to take place in stable environments inhabited by K-strategists, whose biotic adaptations keep their populations on levels where individual birds generally have more food than the bare necessity.