Ardea
Official journal of the Netherlands Ornithologists' Union

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van Balen J.H., Booy C.J.H., van Franeker J.A. & Osieck E.R. (1982) Studies on hole-nesting birds in natural nest sites. 1. Availability and occupation of natural nest-sites. ARDEA 70 (1): 1-24
The scarcity of studies on hole-nesting birds in natural nest sites prompted us to undertake such a study, and to compare the results with those obtained with nest-boxes. The study was performed in 1975-1977 in a wooded area north of Arnhem, where many roads are bordered with old deciduous trees, rich in tree holes. The study area consisted of three sets of roads and a 9 ha plot of deciduous woodland (Warnborn B, see Fig. 1). From the suitable holes found several measures were taken (Fig. 2). Suitable holes occurred in densities of 6-30 per ha, or 3-11 per 100 trees. High densities were found along the roads (Table 2), presumably because the surrounding forest was rather young and unsuitable for hole excavation. Conifers had very few holes (Table 3), but the various deciduous tree species did not differ in hole density. Young trees (trunk circumference smaller than 60 cm) had no holes at all and there was a general tendency for hole density to increase with the age of the trees (Table 4). All measured hole parameters showed considerable variability (Fig. 3), enabling a wide selection of species to nest. Many species were found indeed, but Starling and Great Tit predominated (Table 5). Occupation percentages were high (54-93%), especially in 1975 and 1977. The species composition differed appreciably from that in neighbouring nest box areas (Table 6). These differences can be understood from the differences in size and entrance diameter between nest-boxes and tree holes. Only the striking absence of the Pied Flycatcher in the tree holes could not be explained in this way. The annual fluctuations in the numbers of clutches of the most common species (Table 7) corresponded with the fluctuations found in several nest-box areas (Table 8). The properties of occupied and unoccupied holes were compared for all species together and for the most common species separately, in order to study preferences and interspecific competition. In general, holes with a large diameter, depth, bottom area, and volume were occupied most frequently, as well as holes exposed to eastern directions (Fig. 4 and Table 9). Starlings did not occupy holes with an entrance diameter of less than 3.5 cm. For the remaining holes it was found that those at small heights and those exposed to W-NNW directions were avoided. This also holds for holes with a wide entrance (diameter> 5.5 cm), and for holes with a small diameter, bottom area and volume (Table 10). For the Great Tit we made separate calculations for those holes that were left unoccupied by larger-sized (dominant) species (Table l1b), to enable conclusions about preferences to be drawn. Initially no significant differences were found, but by further pooling of classes it was found that holes at low heights (0-4 m) and holes with a small entrance 3.5 cm) were preferred. An experiment, in which the entrance of tree holes were reduced to ca. 3cm to exclude larger species, showed that in this situation the Great Tits preferred to nest in holes with a large diameter, bottom area and volume. Blue Tit clutches were predominantly found at very small or at great heights, and most of their holes had either a very small entrance or a small hole diameter (Tables 12 and 13). 'These findings were evidently due to interspecific competition. Data for the other species were scanty (Table 14). Coal Tits nested at low heights, in holes with a small entrance and a rather small bottom area. Marsh Tits nested in extremely narrow holes, also with a small entrance. The Nuthatch nested at greater heights than most of the tits. The entrances, often rather large, were narrowed by plastering to a uniform size (2.7-3.0 cm). The bottom area was usually rather large. The habit of narrowing the entrance increased its success in the competition with larger species. Redstart holes were situated at small heights and had a large and rather variable entrance size. The fact that many holes were used by different species in successive years (Table 15) suggests. that the preferences of these species overlap each other considerably and that there is ample opportunity for interspecific competition. Comparison of the numbers and locations of territorial male Great Tits and the numbers of clutches found in the tree holes showed that in 1977 only about 30% of the pairs succeeded in laying a clutch in a hole inside their territory. Competition with Starlings was the main cause of this low success rate. The other tits either stayed without nesting or moved several hundred of metres in order to nest outside their territories. Narrowing the entrances in part of the area resulted in reduced occupation by Starlings and increased occupation by Great Tits (Table 16). In another experiment the entrance of nest-boxes occupied by Great Tits were enlarged during the laying period; this resulted in a rapid take-over by Starlings.


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