OIKOS 89: 41–45. Copenhagen 2000 Does predation maintain tit community diversity? Cecilia Kullberg and Jan Ekman Kullberg, C. and Ekman, J. 2000. Does predation maintain tit community diversity? – Oikos 89: 41–45. European tits of the genus Parus constitute a complex group of coexisting boreal birds. Here we present a survey of the distribution of three coniferous-living Parus species and one of their main predators, the pygmy owl (Glaucidium passerinum), on nine isolated islands in Scandinavia. On all islands the coal tit (Parus ater ) is the sole tit 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 pygmy owls are present. 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-safe foraging sites. The smaller coal tit is superior in exploitation competition for food, while the two larger species have an advantage in interference competition for predator-safe foraging sites. The association between the 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. C. Kullberg, Dept of Zoology, Stockholm Uni., SE-106 91 Stockholm, Sweden (kullberg@zoologi.su.se).– J. Ekman, Eolutionary Biology Centre, Population Biol - ogy, Uppsala Uni., Norbya ¨gen 18D, SE-752 36 Uppsala, Sweden. Coexisting congeners usually differ in morphology and resource use. One explanation for the non-random combination of trait values among sympatric congeners is that competition is a structuring force behind the composition of species assemblies (Lack 1971). Food depletion will result in mutual competition among spe- cies exploiting a common resource, and segregation in food use is a mechanism that allows coexistence. How- ever, the outcome of interspecific competitive relation- ships involving food may shift depending on the exposure to predators when energy gain is traded against predator protection (Kotler and Holt 1989). Selective predation could then promote coexistence and maintain high diversity of prey species (Paine 1966, 1971). The presence of a predator, which selectively attacks species superior in exploiting food, will release food resources and allow coexistence by species more efficient at avoiding predators. It is rare for more than two congeneric species to coexist in sympatry (Lack 1971). The Old World parids (genus Parus ) are exceptional in the large number of congeneric species that coexist without habitat separa- tion (Haftorn 1956). Niche separation between species within the Parus genus has been subject to extensive studies to understand the basis for coexistence (e.g. Snow 1954, Haftorn 1956, Lack 1971, Hogstad 1978, Norberg 1979, Perrins 1979, Hererra 1981, Alatalo et al. 1986, Ekman 1986, Suhonen 1991, Suhonen et al. 1994, Wiggins and Møller 1997). Within habitat, coex- isting species are separated in foraging niches by using different height portions of trees and different distances to the trunk while foraging (Haftorn 1956). Niche expansion in response to the absence of competitors provides compelling evidence that current competition constrains niche use (Alatalo et al. 1985, 1986, 1987). Here we will focus on the distribution of three species inhabiting coniferous forest of Scandinavia. The two larger species, the willow tit (P. montanus; mass 11 g) and the crested tit (P. cristatus ; mass 11.5 g) forage closer to the trunk than the relatively smaller coal tit (P. ater ; mass 8.5 g) (Haftorn 1956). On several islands around Scandinavia this community of tit spe- Accepted 30 July 1999 Copyright © OIKOS 2000 ISSN 0030-1299 Printed in Ireland – all rights reserved OIKOS 89:1 (2000) 41