Diversity and abundance of large tree holes used by Tawny Owls Strix aluco in lowland temperate forests

Abstract

ABSTRACT Capsule Tawny Owls Strix aluco used a wide variety of tree holes for roosting, but only a minority were used for nesting, which comprised those holes at greater heights in larger trees and with a flat base. Aims We aimed to determine the type and dimensions of tree holes used by Tawny Owls for roosting and nesting, including which tree species were the most important hole providers, and the density of tree holes available for Tawny Owls in forests of different age classes. Methods We conducted visual searches for all large tree holes within study plots, recorded their measurements and signs of use, comparing the features of used and unused holes. Data were collected in medium-aged and old-growth forests (Ukraine, Homilsha Forest, 750 ha, 10 seasons) and in primeval forest (Poland, Białowieża Forest, 150 ha, 2 seasons). Results Roosting holes came in a wide variety of shapes and sizes, frequently being very deep and with an uneven base. Hole entrances needed to be over 10 cm wide and the hollows more than 20 cm deep, presumably to enable owls to enter and evade predators. All nesting holes had a flat base and were located in larger trees, at greater heights than roosting holes. Old trees of low commercial timber value (European Hornbeam Carpinus betulus, Small-leaved Lime Tilia cordata, European Pear Pyrus communis) were the most important hole providers. Suitable holes were found in trees of at least 80 years old, and their abundance increased with tree age, exceeding 50 holes/km2 in the primeval forest. Conclusion Despite the abundance of suitable roosting holes, nesting holes remained a rare resource associated with larger, older trees. As tree species of low economic value were the main hole providers, these should be retained in forestry operations.