Ardea
Official journal of the Netherlands Ornithologists' Union

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Drent P.J. (1987) The importance of nestboxes for territory settlement, survival and density of the Great Tit. ARDEA 75 (1): 59-71
From 1983 onwards a study on the importance of holes and their substitutes (nestboxes) on territory settlement, survival and reproduction has been carried out in different habitats, poor in natural holes. Territory settlement by juvenile males in September, which is the crucial period for territory density in years with a seed food supply in winter, appeared to be independent of the presence of nestboxes. The density of newly settled territories was determined by the density of adult territory owners and juvenile non-territorial males. Occupation of vacant areas during winter and spring depended on the presence of non-territorial birds. Marked shifts in the position of the area were only observed in males with territories in sub areas without nestboxes and were directed towards areas with an excess of nestboxes. Earlier experiences with these boxes for breeding and roosting seemed to be important. In contrast to areas with nestboxes, most territory owners in areas without boxes roosted from October onward outside cavities in their territories. While in areas with nestboxes all territorial pairs used them for breeding, most of the territorial pairs from areas without nestboxes made breeding attempts in nestboxes outside territories and outside the area. This caused marked differences in breeding density and in the density of fledglings. The local survival rate of territory owners was markedly lower in areas without nestboxes than areas with nestboxes. This was due to emigration in summer and winter and to mortality in winter caused by predation, competition for nestboxes and unfavourable weather conditions. The difference in the local survival rate between the areas caused a difference in composition according to age and duration of presence, in years with an excess of non-territorial birds. Moreover it caused a difference in territory density in spring in years with a shortage of non-territorial males. From these results it is concluded that although Great Tits take into account the proximate value 'of cavities for roosting and breeding, the distribution of nestboxes (or natural holes) did not directly determine territory settlement and density. Territory settlement and density were dependent on the densities of resident territory owners and non-territorial candidates. These densities were strongly influenced by the distribution of nestboxes that caused differences in local survival rate and reproduction. Hence, the distribution of nestboxes indirectly determined the territory density.


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