A long-term study of Willow Tit in Finland has revealed that forest management is the key driver behind the observed decline.
The research, which was conducted in a 25-sq-km study area near Oulu, showed that tree felling for forestry has had a cumulative negative impact on the nesting of Willow Tit for at least 30 years.
Only a few decades ago, Willow Tit was considered Finland's fourth-commonest breeding bird, but it is now classified as Endangered due to a rapid decline in recent years.
Willow Tit has rapidly declined in Finland (Jon Mercer).
Willow Tit, along with other similar species including Crested Tit, are dependent on natural forests to be able to prosper. The species nests in dead or dying trees which are still standing, in which they can excavate cavities in the soft decaying wood. Forestry practices, which include clearcutting and thinning, mean that sections of forest are either totally or partially removed, taking with them potential nest sites. Put simply, if there are no suitable deadwood nest sites to use in a forest, Willow Tit cannot breed there.
The research showed that the higher proportion of forest felled less than 30 years ago there was, the lower the number and density of Willow Tits. At the study site near Oulu, breeding density decreased by almost half over the study period. Of this decrease, up to 65% is explained by clearcutting and thinning. The team found that the impact of clearcutting was much greater than that of thinning.
The researchers concluded that the survival of Willow Tit, as well as similar species that depend on old and mature forests, can be improved by avoiding clearcutting. This practice, which involves the complete removal of trees from a section of forest, causes direct loss of habitat for many years, even if the forest is regrown. Satu Kumpula, doctoral researcher at the University of Oulu, explained: "If all the decaying deciduous trees that are important for Willow Tit are also thinned out or they fall, it may cause a decades-long shortage of nesting trees."
The research results show that plentiful amounts of decaying deciduous snags improved the situation and shortened the distance between nests. According to researchers, the presence of dead or dying trees should be taken into account in forestry, because if there are no decaying deciduous trees in a forest, Willow Tit cannot nest there. The amount of decaying trees in forests could also be increased through restoration. Kumpula added: "Willow Tit rarely nests in a nestbox, meaning that these will not be of any help."
It has also been speculated that increases in the populations of Blue Tit and Great Tit may also be having an impact on Willow Tit's decline in Finland, but further study is needed in this area.
Reference
Kumpula, S, Vatka, E, Orell, M, & Rytkönen, S. 2023. Effects of forest management on the spatial distribution of the willow tit (Poecile montanus). Forest Ecology and Management. https://doi.org/10.1016/j.foreco.2022.120694
