Does predation maintain tit community diversity?

European tits of the genus Parus constitute a complex group of coexisting borealbirds. Here we present a survey of the distribution of three coniferous-living Parusspecies and one of their main predators, the pygmy owl (Glaucidium passerinum), onnine isolated islands in Scandinavia. On all islands the coal tit (Parus ater) is the soletit species when the pygmy owl is absent. The two larger species, the willow tit (P.montanus) and the crested tit (P. cristatus), only coexist with the coal tit when pygmyowls are present. We suggest that the coexistence of willow tits, crested tits and coaltits is the result of a combination of competition for food and predator-safe foragingsites. The smaller coal tit is superior in exploitation competition for food, while thetwo larger species have an advantage in interference competition for predator-safeforaging sites. The association between the distribution of the pygmy owl and the twolarger tit species on isolated islands in Scandinavia is consistent with the idea that thepygmy owl is a keystone predator.

Cecilia Kullberg and Jan Ekman, OIKOS 89: 41-45. Copenhagen 2000