Effects of forest management on the spatial distribution of the willow tit (Poecile montanus)

Abstract

Modification, fragmentation and loss of boreal forest habitats have been intensive during the last century due to forestry practises and land use. This has been related to population declines of many forest species, yet the mechanisms affecting on the background are largely unknown. The willow tit, a primary cavity-nesting species that was once the 4th most common bird species in Finland is nowadays endangered. Earlier findings suggest that the willow tit population is affected by the reduction of nesting sites, decaying snags in forests and the loss of mature forests which contain the food storages during the winter. In this study we are searching for the mechanisms how the forest management methods could explain the decline of the willow tit population. We used long-term breeding data of the willow tit nesting sites from 1990 to 2020 collected in a study area in northern Finland to analyse if forest management affected nearest neighbour distances and natal dispersal and breeding dispersal distances. We used Geographic Information System (GIS) methods to combine the ecological breeding data to accurate spatial forest management and habitat quality data. The data was analysed with linear mixed models. We found that clear-cuttings affected the willow tit dispersal and neighbouring nest distances more than thinnings. Both clear-cuttings and thinnings increased the nearest neighbour distances. The natal and breeding dispersal distances were lengthened by increasing proportions of clear-cuttings. The habitat loss caused by clear-cuttings and the decrease in habitat quality caused by thinnings has had a major role in the decline of the willow tit population. The forest management actions were estimated to explain about 65 % of the willow tit breeding density decrease in the study area. The effects of forest management were witnessed in a cumulative 0–30-year period meaning that forest management causes long-term habitat degradation and loss. Availability of deciduous snags in the forests can compensate the habitat loss to some extent by providing better breeding opportunities. As the effects of clear-cutting were more severe to the willow tit than thinning, we recommend using other forest management methods than clear-cutting as the main management method.