Abstract
1. Breeding distributions of 11 woodpecker species in the 98 Japanese Islands and neighboring islands were analysed from the ecological viewpoint.
2. The number of breeding species tends to be larger in larger islands.
3. Among the five species which breed both in the mainland and outlying islands, the occurrence in the outlying islands differs in relation to the size and remoteness of islands.
4. The two species coexisting in an outlying small island are those which are markedly dissimilar in their body sizes and ecology. The body size ratios of larger to smaller species are more than 1.4, 1.8, 3.4 in the wing length, culmen length and body weight, respectively.
5. The nonexistent combinations such as D. leucotos/D. major, P. awokera/D. leucotos or P. awokera/D. major are considered to be prevented from occurring by the ecological incompatibility between the two species, not by the absence of a chance to meet each other.
6. Triplet-type combinations are also those which are effective in reducing the competition to a minimum in an island.
