Ardea
Official journal of the Netherlands Ornithologists' Union

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Drent P.J. (1984) Mortality and dispersal in summer and its consequences for the density of Great Tits Parus major at the onset of autumn. ARDEA 72 (2): 127-162
Numerous studies. have shown that the breeding density of the Great Tit Parus major differs from year to year but that the level around which the density fluctuates is characteristic for the habitat. The mechanisms underlying the differences in breeding density. have not Yet been clarified. The aim of the present study is to analyse these mechanisms by following the fate of individual birds. The study was performed in the areas; Vosbergen Estate (near Groningen) and adjacent Park, and Hoge Veluwe (near Arnhem). In the present paper, a part of this study, the mechanisms are analysed which determine the differences in density of both adults and juveniles at the onset of autumn, which are important for the determination of density in the following spring (Drent 1983). The levels of breeding density in the areas of study and the differences between the years are related to the abundance of oaks and seed food in the preceding winter (beechmast crop and artificial food supply). Each year a number of the breeding birds are so-called guest birds (intruders). The proportion of guest pairs is particularly high after winters with an abundant beechmast crop, thus in years with a high breeding density. Almost all guest parents and their young emigrate directly after the breeding season. The local survival rate of territorial birds is high and differs hardly between the two areas of study. Therefore, the differences in the density of territorial breeding birds are found back in the density of adults at the onset of autumn. The density of fledglings is positively related to the density of breeding pairs. Moreover, in oak-rich habitats more young fledge per breeding pair, due to the high survival rate of the nestlings. The proportion of males among the fledglings is negatively related to breeding density and positively to nestling mortality. The local survival of the fledglings until 1 September differs between the habitats and between the years. The more oaks and the higher the survival rate of the nestlings and their fledging weights, the higher the local survival rate. When dispersal of juveniles is taken into account, the total (= real) survival rate until 1 September amounts to 40-50 per cent in both areas and in almost all years. Differences in the mortality rate arise in the period shortly after independence. This mortality affects particularly light juveniles, especially females. The total survival rate is positively correlated with the annual mean fledging weight, but neither with the density of adults and juveniles, nor with the proportion of second-brood juveniles. In a year with a relatively low total survival rate the first broods with a low mean weight and an early fledging date have the highest losses. Nevertheless these two factors explain only half the variation in the survival rate between broods. Since, irrespective of the mean fledging weight, the survival of successive broods of the same parents (first and second brood in the same year, first broods in successive years) is positively correlated, an important role in determining the survival chance is attributed to the characteristic timing of 'parental meanness'. The major differences in local survival rate are caused by variation in dispersal. Two different types of dispersal can be distinguished. Firstly, directly after fledging, juvenile experiences of the parents and the distribution of oak patches over the adjacent areas cause a redistribution of broods favouring habitats with many oaks, particularly those in which the seed food had been abundant in the preceding winter. The consequences of this redistribution are discussed with respect to the so-called buffer-mechanism of breeding densities. Secondly, shortly after gaining independence juveniles start to disperse individually. The individual dispersal rate is correlated negatively with the annual mean fledging weight and with the abundance of oaks. Towards September the emigration rate from areas with many oaks increases while the immigration rate decreases. This runs parallel with a decrease in attractiveness of oaks for foraging. As a consequence of the difference in dominance status the change in dispersal rate particularly affects females. In the. course of time males become more and more sedentary. The shift towards more emigration from the oaks results in a decrease of the marked differences in the density of juveniles between adjacent oak-rich and other forests as found shortly after independence. Concluding, the differences in density of juveniles in oak-rich and adjacent other habitats on 1 September are much larger than can be expected from the reproductive rates and the total survival rates. This is mainly a result of dispersal by broods in the dependent period. Any differences in juvenile density at 1 September will be found back in differences in territory density in autumn and hence in spring. The consequences of the present results are discussed in the light of the controversy between Kluyver and Lack.


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