The Evolution of Nest-Site Selection among Hole-Nesting Birds: The Importance of Nest Predation and Competition

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Abstract
Predation on nests in natural tree holes and in nest-boxes was studied in deciduous and mixed deciduous/coniferous forests at Stenbrohult, South Sweden, during 1972-1980. Predation was the major cause of total nest failure. Predation rates on nests in natural holes were higher than in nest-boxes for the great tit Parus major and the pied flycatcher Ficedual hypoleuca, while no significant difference was found for the blue tit P. caeruleus and the marsh tit P. palustris. In tits, woodpeckers were responsible for 17% of the predation on nests in natural cavities but for 48% on nests in boxes. Of nests that were preyed on, woodpeckers destroyed a lower proportion of those of the great tit than of those of the blue tit and the marsh tit. Minimum nest entrance widths were correlated with the size of the species. Depths of nesting holes were generally similar for different species, but the blue tit occupied shallower holes than did the great tit. The starling Sturnus vulgaris, the nuthatch Sitta europaea, and the blue tit occupied holes higher up in the trees than did the great tit and the pied flycatcher, in accordance with results of other studies. Marsh tits nested very low. Total rates of nest failure and predation were greater in low nests than in higher ones for the starling, the blue tit and the marsh tit. Differences between species in nest predation rates in natural holes could be explained by the vulnerability of the nest sites to predators. For the starling, the nuthatch and the blue tit average nest heights were negatively correlated with their respective population densities. For the marsh tit there was no such correlation, but its nest heights correlated negatively with the density of superior nest competitors. Independent nest preference and utilization data suggest that all 4 spp. that vary their nest heights in relation to density prefer to nest high. This indicates that there is competition for safe nest sites. The preference for nest sites higher up in the trees is adaptive and predation is probably the factor that has selected for this preference. The starling reduced the breeding success of the nuthatch by taking over holes occupied by the latter. The proportion of nuthatch nesting attempts that failed because of such interference was higher in high nests and when the population density of the nuthatch was high.