The five warblers do not seem to have special nesting space or nesting material requirements which would necessitate a larger amount of space than food gathering. Territory defence is probably the only activity of the warblers that requires
an area of comparable size to that needed for food gathering. As discussed earlier, territoriality may exert a limiting effect upon populations under conditions of abundant food supply, thus acting as a stabilizing factor. However, if territory re
quirements limit populations under normal conditions, it may be inquired why natural selection has not reduced territory sizes thus permitting larger populations. Furthermore, variation in warbler population density from plot to plot suggests that more than an incompressible territory is responsible for population regulation. Therefore,like nesting space and nesting material requirements, territories probably require less space for warblers in normal years than does food gathering. Consequently, suitable space is probably the amount of space with an adequate food supply in
which the bird is adapted to feed. Direct measurement of the food supply would require a very elaborate sampling scheme. However, measurement of the amount of foliage of the type in which the species has been shown to feed is quite feasible.If the density of breeding pairs of a species is proportional to the amount of foliage in a certain
zone, then a census of a plot with twice the volume of this foliage should have twice the number of birds of corresponding species. Since the five species under consideration feed above 20 feet,only foliage above 20 feet was measured.