The natural tree holes and nest holes of hole-nesting birds were surveyed in four forest types in the
west Khentii Mountains of NE Mongolia. The utilization patterns of species, size and condition of trees, as well as
hole types, were investigated. The average density of tree holes in the study area approached 30 holes/ha, while
that of hole-nesting birds was 2.4 nests/ha only. The riparian mixed forest had the greatest number of species and
individuals of hole-nesting birds, while the spruce-fir forest had the lowest numbers. Excavating bird species
preferred larger, deciduous trees, and snags. Non-excavators did not select holes according to tree species or
size, but preferred holes in living trees and branch holes. In view of the low occupancy of holes among the four
habitats, we suggest that the density of secondary hole-nesting birds is not limited by availability of holes in the
study area