|
Résumé :
|
[Populations of great and blue tits (Parus major L. and P. caeruleus L.) have been recorded since 1947 in Wytham Wood, near Oxford; this paper is mainly concerned with the results on the great tit from 1959 to 1963. The numbers of the two species tend to fluctuate in parallel with each other and with the numbers in other populations. The breeding season is short, the majority of the birds having only one brood. The date of laying has varied by over a month in different years, this fluctuation being in parallel with that of some of the more common caterpillar species. It seems probable that both tits and caterpillars are affected by a third factor, possibly the opening of the oak leaves, since there is a good correlation between the mean date of laying and the temperatures in spring. The mean clutch has varied between 12.5 and 7.8 in different years. It is suggested that the mean clutch is usually that which produces the most surviving young, though this is not always so. Four sorts of variations in clutch-size are shown, namely that clutches are smaller later in the season, smaller at higher densities, smaller in habitats with fewer large trees, and smaller when laid by birds which are breeding for the first time. These are all explained on the basis that by laying a smaller clutch when the chances are not so good for raising young the parents are more likely to raise some healthy young which will survive to breed than if they had a larger clutch; hence such modifications have been produced by natural selection acting on the individual through the number of progeny it leaves. It seems that most of the young tits are normally dead before the winter; the number that are still alive at the beginning of winter is usually a good indication of whether there will be an increase or a decrease in the numbers breeding the next year. The numbers of tits are also influenced to a considerable extent by the size of the beech-mast crop. If there is a large crop more birds survive than if there is not. This may be partly because many other crops fluctuate in parallel with those of the beech. Attention is drawn to the fact that the earliest breeding birds produce the most surviving offspring. Discussion centres on why, if this is so, most great tits do not breed earlier than they do. The only likely solution seems to be that the birds are unable to get sufficient food earlier in the season to produce eggs. It is considered that the great tits in Wytham are, usually, rearing as many young as they are able to and that the numbers are greatly influenced by the food supply, both immediately after the breeding season and during the winter.]
|
