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Habitat selection of secondary hole-nesting birds in riverine forests along Drava river in Croatia
Izbor staništa sekundarnih dupljašica u poplavnim nizinskim šumama uz rijeku Dravu u Hrvatskoj
Mario Slatki, Jelena Kralj
The correlation between secondary hole-nesters community characteristics and floristic and structural characteristics of their habitat was studied in riverine forest stands near river Drava in Croatia. Standard point count method was used for bird community sampling and circular plot method for habitat sampling. Sampling was carried out on 66 points. PCA analysis that included 28 independent habitat variables was used, followed by Spearman rank correlation between principal component scores and bird community variables (number of species and number of pairs). Tree basal area was used as an indication of stand maturity and to classify studied points into four forest types (ash, poplar, alder and mixed). Eight secondary hole-nesting species and 14 tree species were recorded. The average forest age was 59.8 ± 20.5 years, with ash and mixed stands being on average older than alder and poplar stands. Shannon-Wiener index of secondary hole-nesters diversity was highest in stands with dominant ash and was increasing with stand maturity. A significant positive correlation was found between number of bird species as well as number of pairs and older stands with lower number of tree species and lower relative number of poplar and alder. It can be concluded that 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.
KEY WORDS: bird community, forest habitat structure, forest age
Hole-nesting species represent an important part of forest bird communities. Although hole-nesters can breed in different substrates, including ground, among rock crevices or even in buildings, the majority of them commonly nest in trees. Many of obligatory hole-nesters are secondary hole-nesters which cannot excavate the wholes. Therefore, they rely on existing holes, produced by woodpeckers or by fungal decay (Newton 1994). In the forest habitats, the shortage of nesting cavities is most important factor for limiting the number of hole-nesting birds (Newton 1994). Holes are mostly situated in rotten or dead wood (Ćiković et al. 2014), therefore the abundance and diversity of hole-nesting birds increase with the forest age (Haapanen 1965). In studies of forest bird communities, it was often discussed whether physiognomic or floristic structure of forest has major impact on population densities of small insectivorous birds, including secondary hole-nesters (Blondel et al. 1973, Mac Arthur and MacArthur 1961, Moskat 1988).
Recent studies dealing with species-specific responses of woodland birds to stand-level habitat show that both the habitat structure and the floristic composition of woodland are relevant when trying to explain the distribution of birds