Ardea
Official journal of the Netherlands Ornithologists' Union

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Carlson A., Sandstrom U. & Olsson K. (1998) Availability and use of natural tree holes by cavity nesting birds in a Swedish deciduous forest. ARDEA 86 (1): 109-119
Cavities in trees are an important resource as nest sites or shelter for many birds and mammals. The aim of this study was to quantify the density of cavities, their characteristics, and their origin, and the implication these have on nest site choice by hole-nesting birds. The study was carried out in a deciduous forest in south central Sweden during two years. The forest was dominated by Pendunculate Oak Quercus robur, Norway Maple Acer plantanoides, Small-leaved Lime Tilia cordata, Rowan Sorbus aucuparia, birch Betula pubescens/verrucosa and Aspen Populus tremula, in decreasing order of abundance. We found an average cavity density of 60.4 ha-1. Limb holes were the most abundant type found (53%) and were also most frequently used by hole-nesting birds (64.8%). Pendunculate Oak and Aspen were the two tree species richest in cavities. Limb holes dominated in Pendunculate Oak while woodpecker-excavated holes dominated in aspen. Cavities with occupied bird nests had narrower entrances, were located higher up, had smaller volumes, thicker walls and a smaller circumference of the stem at the hole compared with unoccupied cavities. Limb holes, woodpecker holes and other hole types were used as nest sites by birds in proportion to their frequency of occurrence in the forest. Each year only 5-10% of available cavities attracted breeding birds. The most salient features that emerge from these results are the high density of cavities and their low occupancy rate


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