Perrins C.M. & Geer T.A. (1980) The effect of Sparrowhawks on tit populations. ARDEA 68 (1-4): 133-142

The fluctuations in breeding numbers of the Great Tit population in the 1960-1972 period on a 320 ha study area on De Hoge Veluwe were analysed. This was done by an analysis of the numerical relations between parameters of the population and some environmental variables. During part of this period extra seed food was provided in the winter. A preliminary analysis showed that the size of the beech crop has a large effect on juvenile survival, and hence on population size; the provision of extra seed food by the observers enhanced the survival of the older age classes in particular. Further analysis by multiple regression and canonical correlation techniques gave following results: - The number of autochthonous yearlings in the breeding population (i. e. young born in the study area in the preceding year) is positively correlated with the number fledged the year before and with the size of the beech crop; moreover, it is negatively related to the number of breeding birds in the preceding year, which indicates that the yearlings' chances of settling depend on the amount of space left vacant by the older birds. The beech crop influences juvenile survival between fledging and first breeding. The number of older breeding birds is affected by the provision of extra seed food, which enhances the survival of this category. Furthermore, there are indications that density-dependent losses (due to mortality and/or emigration) occur, and that the severity of the winter plays a role. - The different responses of yearlings and older birds to the two types of seed food can be understood from the distribution of the food sources, the degree of site-attachment, and the dominance relations of different categories of birds. The fluctuations in numbers, foraging behaviour, and diet in winter are discussed next, on the basis of observations carried out in 1973-1979. In these years the number of breeding pairs fluctuated considerably, which was in accordance with the prevailing feeding conditions and the severity of the winters. In the winter of 1976/77 there was a very good crop of beech mast. Although beech mast was already available in September/ October, large-scale feeding on these nuts only occurred in December-January, and especially on days with frost and snow cover. This suggests that beech mast functions as an emergency food source. The impact of a short spell of unfavourable weather was very distinct in February of 1978, when the number of tits present suddenly decreased by about 80 per cent after a period of snow on the 14th. The absence of a seed food supply and unfavourable winter weather resulted in the largest decrease in breeding numbers (46%) found in this study. A similar situation occurred in the winter of 1978/79, but now the unfavourable period started earlier and lasted much longer. During this period a large part of the study area was evacuated by day, and to a lesser extent by night. A large part of the population then foraged at feeding stations outside the area. The area was repopulated in March, and the overall decrease in breeding numbers only amounted to 24 per cent. The tits that foraged in the area from September to December, and also in March, consumed mainly animal food. This too indicates that animal food is preferred, and that seeds serve as emergency food. It is shown that part of the disappearance of the yearlings already occurs before November; on the other hand, processes that determine. the size of the breeding population act throughout the winter. There are regional differences in the extent to which beech mast is consumed in the autumn and winter. In southern Scandinavia the Great Tit seems to be much more dependent on beech mast than is the case in The Netherlands and (possibly) in England. The impact of severe winter weather on Great Tit populations differs considerably between countries in relation to the general climate and the feeding conditions.