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ŠUMARSKI LIST 11-12/2020 str. 27     <-- 27 -->        PDF

old forest cover and trunks with a large circumference (Suorsa et al. 2005).
Hole-nesting birds in general show preference to older forest stands and structural characteristics related to forest age (ratio of large trees and average tree basal area) have the most pronounced effect to the densities of different ecological groups of birds (Kirin et al. 2011). This was confirmed by studies showing that size of trees and abundance of dead trees are important habitat factors especially for hole nesting birds (Berg 1997, De Zan et al. 2016, Suorsa et al. 2005). This was also the hypothesis which was confirmed in our study. Our analysis of the biodiversity index of secondary hole-nesting birds in different age groups shows an increasing abundance of birds with stand maturity, which is also is in line with previous study of Blondel et al. (1973) and Mac Arthur & Mac Arthur (1961).
In our study, the floristic composition of the forest also had an impact on secondary-hole nesters. The importance of floristic composition was also found by Moskat (1988) in beech forest bird communities in Hungary.
In Southwood’s (1961) study of insect species associated with various trees, oak trees had the greatest number of insect species. Many hole-nesting birds feed on insects such as caterpillars during the breeding season (Moeed 1980, Török 1986, Wilkin et al. 2009) so trees with higher diversity of insects should be favourable for them. In our study, forest stands classified as mixed have a large ratio of pedunculate oak in eight counting points which can explain their higher diversity index (Figure 3). On the other hand, our study showed lower abundance and number of secondary hole nesting species in stands with dominant poplar and alder. This can be related to their age, since average stand age was lower in stands with dominant poplar and alder (Figure 4) or with the floristic structure (as number of tree species was lower in poplar and alder stands compared to mixed stands). The effects of floristic structure and forest age should be further studied, to provide valuable data for establishing sustainable principles in forestry management.
It can be concluded that our hypotheses are confirmed. We showed that in riverine forests, diversity of secondary hole-nesting bird species as well as their abundance is correlated with structural habitat characteristics and that older stands show greater bird biodiversity and abundance.
We would like to thank Hrvatske šume d.o.o. for providing data on forestry management in Croatia and for helping out during the field work by allowing access to counting points. We also thank two anonymous reviewers for their valuable comments.
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