Multi-scale settlement patterns of a migratory songbird in a European primaeval forest

Abstract

The drivers of animal settlement are core topics in ecology. Studies from primaeval habitats provide valuable but rare insights into natural settlement behaviour, where species are unconstrained by habitat fragmentation and modification. We examined whether territorial male songbirds (wood warblers Phylloscopus sibilatrix) exhibited clustered distributions when settling in extensive primaeval forest. We tested whether settlement patterns were consistent between years, the influence of habitat preference and the spatial extent of these processes. Remote sensing (airborne LiDAR and satellite multi-spectral) and field survey data were combined to investigate settlement at the landscape (tens of km2), plot (46–200 ha) and microhabitat (20 m diameter) scales, by sampling across 5.9 × 4.5 km of the Bialowieza National Park (Poland) during 2018–2019. We assessed bird distribution patterns using nearest neighbour analyses, and habitat preference with generalized linear mixed models. Variation in bird density between plots and years was suggestive of large-scale clustering at the landscape scale. At the local plot scale, any clustering was masked by territoriality, with birds appearing dispersed or randomly distributed. At the microhabitat scale within territories, birds preferred a tall, closed canopy forest structure and avoided a dense herb layer of wild garlic patches. Settlement appeared unconstrained by the availability of preferred habitat at the plot scale, which remained unsaturated. Wood warbler settlement apparently involved hierarchical decisions at multiple spatial scales, compatible with potential social attraction, territoriality, habitat preference or predator avoidance. Selecting an appropriate scale of analysis is an important consideration when assessing the clustering of territorial species. Many songbirds settle in clusters of breeding territories, possibly reflecting patchy habitat quality and/or social attraction. Studies of territory clustering generally come from fragmented, modified habitats, such as secondary forest, which might bias or limit bird distribution. We studied settlement patterns of male wood warblers in an extensive, primaeval forest, where they could exhibit natural behaviour. Spatial analyses of satellite, airborne and field survey data revealed a series of settlement decisions at multiple spatial scales. We found evidence supporting large-scale clustering across the forest, but at the local scale any such clusters were masked by the spacing of birds within territories. A tall forest structure was preferred within the territories, but at larger scales, some birds seemed to compromise habitat quality to settle within a cluster. The results highlight that settlement decisions of animals must be considered at appropriate spatial scales.