Tree cavity abundance and nest site selection of cavity nesting birds in a natural boreal forest of West Khentey, Mongolia

Abstract

Due to their close association with trees and cavities, cavity-nesting birds (CNBs) are highly sensitive to the alteration of forest structure. Therefore they are considered as indicators of several forest qualities and forest bird diversity, and many CNB species have declined due to modern forestry and anthropogenic landscape alterations.The study site was located in Khan Khentey Strictly Protected Area, northeast Mongolia. The field work was carried out in four different habitats: the mature birch-larch forest, the young birch-larch forest, the riparian mixed forest and the spruce-fir coniferous forest. Habitats were sampled with plotless method. Tree species, DBH, tree condition and if the tree carried fire scars and fungi conks were recorded, and each sampled stem was searched for cavities. Cavities were classified into woodpecker hole, other bird-induced hole, branch hole or bark crevice. Nests of all CNB species were searched in 2002 and 2003. For each nest, the species, DBH, condition and the presence of fire scars and fungi conks of the nest tree were recorded, as well as nest cavity type, cavity height above ground, substrate diameter, substrate condition and cavity opening length and width.The cavity abundance in the study area averaged 30 cavities/ha. The spruce-fir forest had lowest cavity density (18 cavities/ha), due to less branch holes in this habitat. Over 50% of the cavities were found in birch, while poplar was the most cavity-rich tree species related to its abundance. Cavity holding rate rose with ascending DBH. Cavities, especially excavated ones, occurred overproportionally in tall snags. Both the occurrence of fire scars and fungi conks showed positive association with the occurrence of cavities.The density of CNBs in the study area averaged 2.4 nests/ha, lower in the spruce-fir forest than in other three deciduous-dominated forest. The overall cavity occupancy in the study area was approximately 5%. Deciduous trees, larger trees, dead trees and trees with fire scars and fungi conks were preferred by primary cavity nesters (PCNs). Secondary cavity nesters (SCNs) also used such trees overproportionally, which mostly in accordance with cavity availability in these trees.For individual species, the nest sites of D. minor, P. montanus and F. albicilla had high similarity, as they nested in excavated cavities with small and round openings located high in thin and broken snags. The nest sites of D. major and S. europaea were similar in that they both bred in cavities with round openings of diameter 4-5 cm located high in large and living or intact dead trees. Both P. ater and P. major preferred branch holes with slit-like openings, located low in living trees. P. auroreus utilised a wide range of cavities, while C. familiaris was distinct from others in specialising on long slits.The observation of sequential cavity use showed that the cavities previously occupied by D. major and P. montanus were frequently used by S. europaea and F. parva, respectively, in the following year. Previous S. europaea and P. ater cavities were frequently reused by the same species. Cavities in living substrates were reused more often than those in dead ones. Nest web analysis suggested that no SCN species was completely dependent on middle- or small-sized woodpecker holes. They could switch to branch holes when woodpecker holes were lacking. The removal of birch would, however, strongly influence the structure of CNB community.