Popular Science: Do birds in our forests fare well or not?
The authors assembled long-term data on environmental change (atmospheric CO2 concentrations, N depositions), economic factors (timber prices, GDP) and forest management (timber harvest, fraction of mixed forest area) from Germany. Bird abundance data was collected during the long-term monitoring of common species in central Europe – Austria, Belgium, the Czech Republic, Germany and Slovakia. The authors analysed data concerning forest bird species that have breeding populations in these countries, especially non-migrant species.
Why birds? The European Union has designated birds as a suitable bioindicator of the effects of changing land-use. Birds are one of the top predators of invertebrates, therefore they should rapidly respond to changes in habitat and food availability in the environment. Moreover, thanks to their size and mobility, birds may also reflect changes on a wider environmental scale.
Environmental and economic factors
The level of atmospheric CO2 in Germany indicates ongoing emissions of greenhouse gasses, which can in turn influence rainfall distribution, winter temperatures and the length of the growing season. On the other hand, pollution (the rate of N-depositions in forests) has decreased since 1970.
Even though GDP shows a steady increase, timber prices do not increase, but show major oscillations. The harvest of forest wood was not correlated with technological developments or timber prices. Instead, it reflected the current political or ecological situation – the harvest peaked during World War II and after high wind events. The authors were also able to analyse how forest management has changed during the past several decades. Forest monocultures are being replaced by mixed forests with unevenly-aged trees. These forests now represent two thirds of the forests in Germany. Even though the amount of forests that are protected increased from 1% to 4% in the last decade, they still probably have only a minor effect on bird populations, because the area is too small and mixed forests also tend to be aggregated.
European forest bird populations
The study shows that the abundance of non-migratory forest specialists in Central Europe is increasing. A thorough analysis of data revealed a strong influence of changing environmental conditions and forest management on European forest specialists (these factors explained 92% of the variations in the studied bird populations, with both factors being of similar importance).
Migratory birds, on the other hand, show a different pattern. Species that overwinter in Africa are in decline and species that migrate regionally (within Europe) show constant abundances. Both continentally and regionally migrating birds were shown to respond negatively to environmental changes (some migrating species currently have problems coordinating their migration due to climate change, which leads to the decline of migratory bird populations) and to a lesser extent also to changes in forest management.
Other intriguing questions arise here. Is, for example, forest bird abundance related to changes in winter habitats (the length of the growing season, etc.)? And what about interactions between resident forest specialist bird species and migratory species? Do they play any role? The model these authors used, however, cannot answer these questions.
Forest bird populations in the USA
Europe and the northeast USA are ecologically similar and they have experienced a similar history of pollution and deforestation. This study shows that the trends described above not only reflect the situation in Europe, but they are quite similar to the situation in the northeast USA. In the USA, however, the increase in forest bird species is somewhat slower.
The authors were also able to detect several species that responded differently to changing conditions:
From non-migrating birds, the Lesser Spotted Woodpecker (Dendrocopus minor) had high abundances in the 1980s but then declined. Bullfinches (Pyrrhula pyrrhula) have also declined.
Among regionally migrating species, Stock Doves (Columba oenas) increased, probably because they use old nesting holes made by woodpeckers and also due to increasing maize cultivation (they feed on maize regularly). Common Chiffchaffs (Phyloscopus collybita) profit from an increasing proportion of mixed forests and thus their abundance also increased.
Only one of the studied continentally migrating species has increased - the Common Redstart (Phoenicurus phoenicurus). However, its numbers were quite low in the late 20th century.
Do birds in open land also fare well?
The situation on non-forest land is probably quite different from that of forests. A decline of non-forest bird species has been shown to be linked to a severe reduction on insect biomass, habitat loss and urbanisation. In the UK, for example, it has been shown that insect abundance declined only in open habitats. Moreover, the ongoing climate change might have a positive effect on non-migratory forest birds due to longer growing seasons and less severe winters. However, these effects are unlikely to persist in light of expected larger changes in climate.
The study has also shown that sustainable forest management (mixed age-uneven forests replacing monocultures) can have long-lasting and positive effects on biodiversity. However, it is important to note that “no single practice of forest management can provide suitable habitat conditions for the conservation of all forest bird species”.
Schulze, E. D., Craven, D., Durso, A. M., Reif, J., Guderle, M., Kroiher, F., ... & Eisenhauer, N. (2019). Positive association between forest management, environmental change, and forest bird abundance. Forest Ecosystems, 6(1), 3.