Hutchinson found consistent size differences in competing bird bills. Remson found that a 1.2 size difference seems to be required for kingfishers to occupy same stream--prey sizes correspond to difference of bill size.
Diffuse competition is competition from unclosely related animals, like lizards, frogs, ants, rodents. For example, the Takahe, a large, moorhen-like bird in New Zealand, competes with Red Deer for grass: the deer takes more types of grass and hence is more common. Indeed, Takahe are near extinction.