Ardea
Official journal of the Netherlands Ornithologists' Union

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Klomp H. (1980) Fluctuations and stability in Great Tit populations. ARDEA 68 (1-4): 205-224
1. Fluctuations of the size of breeding populations of the Great Tit are correlated with the variations of juvenile survival from fledging to first breeding. The size of these variations is related to food abundance in autumn and winter. The density level at which the fluctuations occur varies between habitats and it is higher where food is more abundant. 2. The occurrence of density levels without upward or downward trends suggest that the numbers of birds are regulated by density dependent factors. One of these factors density dependent production of young (per pair per year)was established by Kluyver in 1951, but its regulatory significance has long been a matter of discussion. It was shown in 1970 and 1975 by means of simulation techniques that a density dependence of the 'strength' found, gives rise to a stable point with a slow asymptotic approach to equilibrium after disturbance. This finding is corroborated in this paper by means of the construction of a 'reproduction curve'. 3. The evidence from literature on density dependent survival is reviewed. The conditions which will show that this dependence is the result of territorial behaviour are summarized. Great Tits satisfy these conditions to a large extent, with the exception that substantial numbers of non-breeding birds without territories have never been observed. 4. The effect of territorial behaviour on the size of a breeding population has been modeled, and a 'reproduction curve' has been constructed to explore the combined effect of this behaviour and density dependent production of young on the stability of the system. Again, there is a stable point, but the return time to equilibrium is considerably reduced. 5. Some of the relations found in field data between age ratios and population change, between numbers of individuals in a population at different times of the year, and between density and the survival rates of varies categories of birds are predicted by the model In other cases the agreement between prediction and field evidence improved by making the survival rates outside the breeding season density dependent. 6. Several arguments in favour of the occurrence of competition for food in summer, autumn and/or winter are discussed, and it is concluded that an intensive long-term study of foraging behaviour, diets, energy budgets, and food availability - though tremendously time-consuming - is necessary to explain the numbers of tits in different types of habitat.


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