What are the likely causes of changes in the forest bird indicator?
The forest bird indicator has fluctuated through time, but remains close to its value in 1980. There is the suggestion of a decline in Northern Europe and a number of common forest birds are in decline in Europe, though some are increasing. Changes in forest area, composition, age and structure all influence bird community composition and species trends, both positively and negatively depending on the species. In particular, the loss of active forest management, the maturation of forest stock, loss of dead wood, and increased deer browsing in some areas, have been linked to forest bird declines. Other evidence shows that some forest specialists, particularly birds associated with old-growth stands, have declined severely and are threatened by modern commercial forestry practice. A large body of work has focused on birds of boreal forest and their interaction with forestry, but our knowledge of other forests and their bird populations is less good.
The forest bird indicator contains birds that are long-distance migrants. Studies have shown that some long-distance migrant birds have declined more steeply than resident birds. It is not known whether deteriorating conditions on the breeding grounds in Europe, or on passage, or wintering sites in Africa/Asia is responsible for the declines; it seems likely that a combination of factors is involved.
Learn more in the related question “Sampling the same sites every year is fine for farmland that is maintained, but in forests the species composition will change in the course of 25 years due to succession. Do sampling sites have to be replaced if they change strongly?“.