How do birds like it in Czech forests?

More precisely, the study focused on more than 50 species of common forest birds, for which the scientists worked with data from the Czech monitoring system of their breeding populations. For the research, they defined several bird groups according to the habitat associations of each species with respect to standing size, vegetation strata, and species composition of trees. They then calculated annual indices of population change for each species, averaged these across the groups, and the study was made.

The garden warbler
Source: Wikipedie, Author: Steve Garvie

The results speak volumes - the indices for almost all forest birds showed significant increases over the monitoring period. The increasing trends were then particularly marked for habitat generalists (i.e., species with the greatest capacity to adapt). Groups of more specialized species showed more variable population trajectories: they declined for shrubland birds while increasing for birds associated with sparse forest or deciduous trees. Other groups had stable populations, such as those tied to coniferous trees. In addition, the suitability of forest habitat for species nesting in trunk cavities also appears to increase over time.

And what specific forest management practices have such an effect on birds? This is a question that the researchers have also tried to answer. It is evident that these results are consistent with the ageing of forests and the increasing volume of wood in them. These changes in the forest environment are probably the result of a change in the purpose of forest management. There has been a shift from harvesting still young fuelwood to harvesting mature, older wood. This change took place during the 20th century and, as forest stands evolve only slowly, its effects are only now becoming apparent. Forests harvested for timber today contain mainly tall trees with large trunks that provide limited space for shrub vegetation. In contrast, stands originally harvested for fuel were rich in shrub cover. All of this reflects the status of bird populations tied to their preferred habitats.

The graph of the increase in the volume of wood in forests in recent years
Source: the original study

The increase in the number of species tied to broadleaved trees can be explained by the replacement of coniferous by broadleaved ones in certain parts of the country, which also marks a return towards more natural conditions. The currently observed forest ageing is particularly beneficial for birds such as the Brewer's thrush (Turdus viscivorus) or the homing pigeon (Columba oenas), which are expected to experience the greatest increase in population. The impact of forest ageing is also evident in the species of the shrub layer, which is currently declining. This includes the warbler (Sylvia borin), whose populations are on a downward trend.

The result of the study suggests that common bird species are benefiting from the current state of forests in the Czech Republic. However, we should be careful not to consider this conclusion too optimistic. The data included in the study focuses primarily on generalists, already known for their ecological flexibility, and the study omits, for example, endangered species for which further research is recommended. Increasing urbanization or other factors outside the forest environment may also contribute to the expansion of generalists. Examples include species nesting in tree stands in urban areas, such as the common starling (Sturnus vulgaris), whose population has also increased. Moreover, this work shows that bird monitoring provides very important data. In this case, it served as an indicator of the appropriateness of forest management and may provide further enriching results in the future.

Jiří Reif, Alena Jechumtál Skálová, Zdeněk Vermouzek, and Petr Voříšek. "Long-term trends in forest bird populations reflect management changes in Central European forests" Ecological indicators 141, (2022): 109137.

Tereza Žirovnická