Big birds need old trees
The quality of the habitat and especially of breeding trees are key factors in the nesting site selection of forest-dwelling bird species. Even if some types have the same claims in basic features, these can accompany however with further, different, habitat-claims.
Zawadzki et al. (2020) therefore investigated the habitat structure in the vicinity of the eyrie trees of the white-tailed eagle and the black stork. Both species are known to choose large old trees with large trunk diameters as nesting trees. In order to be able to determine the location requirements of the animals, the surrounding vegetation of the nesting trees was examined within a 15 m radius between 2015 and 2019. Data on DBH, tree height, height of the nest above ground, height to treetop base and type of nest placement (on the crown, in the crown, on a branch directly on the trunk or on a side branch slightly away from the trunk) were collected.
For the black stork 16, for the white-tailed eagle 19 nest trees were examined in the northeast of Poland. For comparison, 50 randomly selected adult trees (older than 100 years) were chosen in the study area, which were examined in the same way as the nesting trees.
Home with a lovely view
The authors determine different preferences between the two bird species regarding nesting site selection. White-tailed eagles brood exclusively on pines, that were popular also with black storks. However, these preferred, if possible, old stalk-oaks. Both bird species selected trees, that exhibited bigger circumferences than the coincidentally selected trees. White-tailed eagles, however, preferred old stands with a lighter canopy and lakes near their nesting tree as well as large distances to roads.
The trees in the surrounding stands were of a similar age to the selected nesting tree. Black storks, on the other hand, preferred „veteran“ trees with deep crowns, which are significantly older than the trees in the vicinity of the nesting tree. In contrast to the white-tailed eagle, the proximity to roads was not important for the black storks.
The authors point out that the nesting trees of both species were always older than the average harvest age of trees under forest management. It is emphasized that forestry has a strong influence on the characteristics of forest habitats. Examples of this are the age and structure of the stand, the occurrence of different stages of development, the diameters of trees up to the tree species composition.
In the lower levels, the herbaceous and shrub layer as well as the undergrowth are influenced by the management and machinery used. Thus, many animal and plant species that depend on forest ecosystems are influenced. Many characteristics that determine the presence or absence of bird species in particular are not found in traditionally managed forests. Thus, the richness and diversity of the bird population is reduced by measures that can be traced back solely to human activity.
For this reason the scientists recommend taking small areas up to larger islands out of use, where trees can reach the age and dimensions that are attractive to forest-dwelling birds as nesting trees. Furthermore, some trees with strong horizontal branches as well as relic trees should be allowed to remain untouched in the stand. All these recommendations are key elements of a near-natural and multifunctional forest management.
The work of Zawadzki et al. shows that forests are more than a resource: they are a habitat.
In the past, large bird species in particular have repeatedly been symbolic figures of nature conservation. Their absence or reduction in population has been able to inspire many people to support environmental protection in general. Their return and the recovery of their population were used as a measure of a successful nature conservation management. Species such as the white-tailed eagle or black stork can serve as key or indicator species for healthy forest management.
They show how important it is to see forests not only as an accumulation of individual trees, but to view them as ecosystems and in connection with the surrounding landscape. The need of these large birds for a variety of old trees also challenges the current concept of habitat trees. It is not sufficient to exclude isolated trees that are worthless for the timber market from use; it is rather necessary to understand the old trees as pillars of the forest ecosystem and to allow more of them to grow in stands.
Ultimately, many smaller species – animals and plants alike – can benefit from the commitment to a few symbolic species, and the old, structurally rich forests created as a result are better equipped to cope with disturbances such as drought and storms.