That the number of Great Crested Grebes (Podiceps c. cristatus) nesting in some parts of England is largely determined by the frequency of nesting sites of a certain type can, I think, be proved from the remarkable rapidity they show in colonizing an entirely new water. `A new reservoir in the Midlands, completed in 1928, was filled with water for the first time early in January, 1929. The area of the water is approximately 136 acres and I learn from Mr. G. B. Kershaw that, though the average depth is 7 feet and the depth at the dam 24 feet, there are many acres where it varies from 18 to 24 inches only. `On June 7th, 1930, only eighteen months after the filling of the reservoir, and although the shallow end and sides were still almost bare of reeds and rushes, I counted fifteen nests of the Great Crested Grebe, all at this end in the shallow water, each with an old bird sitting on it, and one more which a pair was building; several of these were only a yard or two apart, and there was in fact a bunch of ten or twelve in a more or less compact group. `Several other nests were placed at intervals round the reservoir, making a minimum total of twenty nests for the water. `Though they nest in plenty on the Cheshire meres, a pair very often has a reed-bed to itself, and I have never seen anything approaching this gregariousness;