Effects of forest fragmentation on reproductive success of birds in boreal forests

Abstract

The breeding success was studied experimentally using artificial bird nests. The pied flycatcher (Ficedula hypoleuca Pal.) was used as a study object when the effects of patch size, edges and vegetation characteristics on breeding success and distribution of individuals were examined. Only a minor influence of stand size and forest edge on nest predation was found. High edge-related nest predation existed only in the agricultural landscape where it was mainly caused by avian predators. In the forest landscape, nest predation concentrated to large stands as a function of an increased area of clear-cuts in the surrounding matrix. This was likely due to the penetration of rodent-eating mammalian predators to large stands to search alternative prey when vole populations crashed in the surrounding clear-cuts. The relationship between the nest predation rate and vegetation characteristics surroundings of a nest site was mainly associated with the visibility of nests. Sharp-designed man-made edges did not differ in nest predation risk from more feathered natural edges. At the macrohabitat level, pied flycatchers preferred large and medium-sized stands and avoided settling in small stands, which were mainly occupied by unpaired males. Males preferred edges of forest stands but avoided nest boxes situated right at the edge. The avoidance of small stands was possibly due to high costs of food search and the lower amount of food available in small stands. The costs of food search may also be essential when breeding in the extreme edge zone, since in these sites the suboptimal shape of the foraging area may increase the parents' flying distances in feeding the nestlings. At microhabitat level, adult pied flycatchers males inhabited territories with high numbers of deciduous trees while yearling males territories with high numbers of pines. The reproductive output was better in territories of adult males than in territories of yearling males. This age-related unequal distribution of males in relation to habitat quality was likely due to male-male contest over territories in which adult males dominate over yearlings forcing them to habitats of lower quality.