BRITISH WARBLERS
same territory. This may have been the cause of a struggle I once witnessed, towards the end of May, between two Lesser Spotted Woodpeckers (Dendrocopus minor) and one Great Spotted Woodpecker (Dendrocopus major); the latter bird defending a decayed tree, in which possibly its nest was situated, against the onslaughts of the former birds. I have recently gained some further evidence upon this point, for I had an opportunity of watching a conflict between a Great Spotted Woodpecker and a pair of Green Woodpeckers for the possession of a hole in an oak tree. The struggle was carried on intermittently for some days, and as far as actual strength was concerned the Great Spotted Woodpecker was no match for its rivals, but remarkably persistent, and although pecked mercilessly whenever, in the absence of the Green Woodpeckers, it took possession of the hole, yet it refused to leave the locality. Struggles between closely allied species may in reality be of more common occurrence, and may exercise a greater influence, than is generally supposed; amongst the Warblers at least there is no doubt that such is the case, for Willow Warblers and Chiff-chaffs on the one hand, and Blackcaps and Garden Warblers on the other, are constant rivals.
The limits of a territory must be influenced by the conditions of existence of the species. Swamps overgrown with the common reed (Arundo phragmites) not being found everywhere, the territory of the Reed Warbler is consequently small, and has, no doubt, been reduced gradually to the minimum size necessary for the welfare of the species. If the dimensions had been similar to those of the territory of the Chiff-chaff, the species would have rapidly disappeared. Hence there is a point beyond which a species in any one particular district cannot increase, no matter how favourable the conditions of existence may otherwise be. Nevertheless it is probable that this point will seldom be reached, for I can well imagine that the very fact of territory being of such importance as to necessitate one male fighting with another
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