Winter birds in forests
Summary
The populations of winter birds in forests have declined during the monitoring period. Since the 2000s, populations have remained relatively stable on average. Changes in the populations of individual species included in the indicator vary. Some species have increased due to milder winters, but some have suffered due to the decrease in the amount of old forests and dead wood.
Status
The status of the indicator is satisfactory. With ninety percent certainty, the populations of winter birds in forests are at least 13 percent smaller than at the beginning of the monitoring period (1959–1963).
Trend
In the 2000s, the indicator has been stable. Over the entire monitoring period (1959–2022), the development of the indicator has been declining, and the populations of winter birds in forests have, with ninety percent probability, decreased by 0.3 percent per year. However, the decline seems to have been faster at the beginning of the time series, before the 1980s.
Among the individual species included in the forest winter bird indicator, there are both clearly increasing and decreasing populations. Of the indicator’s 16 common species, for example, the black woodpecker has become more abundant as a result of milder winters. Particularly strongly declined species include the willow tit and the crested tit, which have suffered from the decrease in the amount of old forests and dead wood. Species that have also declined include the Eurasian pygmy owl (Baroni 2022, Honkala et al. 2023), which favors old spruce forests, and the goldcrest, which has consistently decreased despite the milder winters over the monitoring period (Lehikoinen & Väisänen 2023). The large annual variation in the indicator’s values is due to the great fluctuation in the populations of migratory birds, which for example in the case of crossbills depends on the seed yield of trees.
Significance
The indicator reflects the state of forest biodiversity. Locally, this manifests as the diversity and abundance of forest avifauna. The population development of forest birds indicates the viability of the populations of the species.
The habitat needs of birds are well understood compared to many other species groups, which allows for the identification of species whose population developments are closely linked to the amount and quality of forests. The habitat requirements of bird species that overwinter in forests vary significantly, but typically, these species avoid open areas and do not thrive in clear cuts or young plantations. The habitats of the species covered by the indicator are predominantly in mature forests. Species most dependent on continuous mature and old forests include the capercaillie, Eurasian pygmy owl, three-toed woodpecker, willow tit, crested tit, Eurasian treecreeper, and Siberian jay. However, the habitat requirements of several species are relatively broad.
Nonetheless, fluctuations in bird populations are not often solely explained by changes in habitat condition, but are also influenced by factors such as climate change, hunting, and natural predation pressure. When designing habitat-specific indicators, it is crucial not only to understand the habitat needs of the species but also to identify these constraints. Among the indicator species, the following are currently hunted: hazel grouse, black grouse, capercaillie.
Bird monitoring forms one of the most central and reliable sources of data for biodiversity in Finland (see “Data used” below).
References
Data used
The indicator is based on monitoring counts coordinated by the Natural History Museum of Finland, carried out by volunteers with excellent bird knowledge. The forest winter bird indicator is based on winter bird count data that began in the 1950s. The indicator thus only pertains to wintering species, which are either resident birds, migratory birds, or partial migrants, a significant portion of whose populations winter in Finland.
The indicator includes the following 16 common forest species:
Hazel grouse – Tetrastes bonasia
Black grouse – Tetrao tetrix
Capercaillie – Tetrao urogallus
Northern hawk owl – Surnia ulula
Eurasian pygmy owl – Glaucidium passerinum
Black woodpecker – Dryocopus martius
Three-toed woodpecker – Picoides tridactylus
Goldcrest – Regulus regulus
Willow tit – Poecile montanus
Siberian tit – Poecile cinctus
Crested tit – Lophophanes cristatus
Eurasian treecreeper – Certhia familiaris
Siberian jay – Perisoreus infaustus
Red crossbill – Loxia curvirostra
Parrot crossbill – Loxia pytyopsittacus
Pine grosbeak – Pinicola enucleator
These species are monitored to assess the biodiversity and ecological health of Finland’s forest environments during the winter season.
More information on the data:
Indicator calculation
The forest winter bird indicator is a multi-species index produced by the Finnish Biodiversity Information Facility of the Natural History Museum of Finland. The index combines estimates of individual species’ population developments into a single index figure. This multi-species indicator updates automatically and its index figure represents the ratio to the base year, which for the forest winter bird indicator is the year 2000.
The calculation of the multi-species indicator begins with the assessment of individual species’ population developments. These assessments are based on bird census counts and generalized linear statistical models, which also take into account differences between counting locations and dependencies between successive years.
Estimates for individual species based on sample-dependent statistical models always include some uncertainty. This uncertainty is accounted for and incorporated into the multi-species indicator through Monte Carlo simulations.
The indicator as a table:
More information on the indicator calculation methods:
Ask for further information
Aleksi Lehikoinen
Senior Curator (Luomus), Head of the working Group on Birds
Päivi Sirkiä
Senior coordinator, group manager (Syke), Use of ecosystem information, systematic species surveys