Does predation maintain tit community diversity?

European tits of the genus Parus constitute a complex group of coexisting b oreal birds. Here we present a survey of the distribution of three conifero us-living Pants species and one of their main predators, the pygmy owl (Gla ucidium passerinum), on nine isolated islands in Scandinavia. On all island s the coal tit (Parus ater) is the sole tit species when the pygmy owl is a bsent. The two larger species, the willow tit (P. montanus) and the crested tit (P. cristatus), only coexist with the coal tit when pygmy owls are pre sent. We suggest that the coexistence of willow tits, crested tits and coal tits is the result of a combination of competition for food and predator-s afe foraging sites. The smaller coal tit is superior in exploitation compet ition for food, while the two larger species have an advantage in interfere nce competition for predator-safe foraging sites. The association between t he distribution of the pygmy owl and the two larger tit species on isolated islands in Scandinavia is consistent with the idea that the pygmy owl is a keystone predator.