Reproductive success of three parid species in managed boreal forests

Anthropogenic impacts, such as forestry and land use practices, can deteriorate habitat quality, affecting reproductive success and leading to population decline. The boreal willow tit (Poecile montanus) population has dramatically declined in Finland. In contrast, great tits (Parus major) and blue tits (Cyanistes caeruleus), both of temperate origin, have expanded their ranges and increased in numbers in Northern Finland. It has been suggested that the decline in the willow tit population is mainly caused by the reduction of breeding sites and wintering areas resulting from habitat loss and degradation. We investigated whether forest management influences the reproductive success of willow tits, great tits, and blue tits, including recruitment across years, while considering other factors such as possible intra- and interspecific competition. We combined long-term breeding data (1990-2020) from Oulu, Finland, with spatial data on forest management activities and analysed the data using (generalized) linear mixed-effect models. Results varied depending on the breeding parameter, species, and forest management method. Forestry had no significant effect on clutch size. We found negative effects of thinnings on nestling survival and growth in great and blue tits, with clear-cuttings negatively affecting blue tit nestling growth. However, recent clear-cuttings had positive effects on willow tit nestling survival and growth. Despite these benefits, clear-cuttings near nesting sites were associated with reduced willow tit local recruitment probability, leading to an overall negative impact on the population. These findings may arise from forest management influences on food availability; certain stages following clear-cutting may improve the food availability for boreal willow tit nestlings, yet thinnings may reduce foraging opportunities for great tits and blue tits. While some positive effects of forestry on willow tits were recorded, they may not be enough to offset the well-established negative impacts of forestry on the loss of optimal wintering and breeding habitats.