Nuthatches (Sitta europaea : Aves) in forest fragments: demography of a patchy population

Breeding density, local survival and summer recruitment of nuthatches were evaluated in a population scattered over many small (1-30 ha) forest fragme nts, and compared with study plots inside larger forests. Since most young birds settle outside the fragment in which they were born this population c orresponds to the "patchy population" concept implying that patterns in abu ndance may be better explained by processes at the population level than by metapopulation processes. Mean breeding density was c. 50% lower in fragme nts and decreased with regional isolation (distance from larger forests) bu t not with local isolation (distance to nearby fragments). Local survival o f adults and established (i.e. territorial) Ist-year birds was not related to forest size or isolation. However, fewer young birds settled in summer i n the fragments compared with a large forest. This difference probably refl ects high mortality during the sensitive dispersal phase. Moreover, the obs erved number of recruits and their estimated survival was insufficient to m aintain the breeding population, suggesting significant net immigration fro m larger forests. This "rescue effect" explains why densities are affected by regional, but not local isolation.