In this study competition has been viewed in the light of the statement that species can coexist only if each inhibits its own population more than the others'. This is probably equivalent to saying that species divide up the resources of a community in such a way that each species is limited by a different factor. If this is taken as a statement of the Volterra-Gause principle, there can be no exceptions to it. Ecological investigations of closely-related species then are looked upon as enumerations of the divers ways in which the resources of a community can be partitioned.
For the five species of warbler considered here, there are three quite distinct categories of "different factors" which could regulate populations. "Different factors" can mean different resources, the same resources at different places, or the same
resources at different times. All three of these seem important for the warblers, especially if different places and times mean very different=-different habitats and different years.