Great Tit Parus major survival and the beech-crop cycle

The single most important environmental variable correlating with annual survival of both juvenile and adult Great Tits Parus major is the beech crop index (BCI). This index is a measure for the amount of seeds of beeches present in the winter, and correlates with crop size of several other tree species. Two, not mutually exclusive, hypotheses exist to explain the correlarion between BCI and annual survival . The first is that the amount of seeds directly affects survival (winter-food limitation hypothesis) . To test this hypothesis we re- analysed a supplemental food experiment of Van Balen (1980), extended with three more years of previously unpublished data. We found that supplemental food increased su rvival of both juveniles and adults, mainly in low BCI years, confirming the winter-food limitation hypothesis . The second hypothesis states that winters with a high BCI are preceded by springs with high densities of caterpi ll ars (the breeding-season food limitation hypothesis). Using data of three long term studies of Dutch Great Tit populations we show that this hypothesis does not hold because (1) high BCI winters are not preceded by springs with high caterpi llar densities, (2) annual mean juvenile fledging mass increased with increasing caterpillar densities, but no effect on parental mass was found, and (3) the annual survival of juveniles was not affected by the annual mean fledging 7,l mass, nor was annual adult survival affected by their mean mass during hick rearing . Finally we show that, despite the fact that the level of BCI probably can be predicted at the time of the breeding season and that both juvenile and adult survival is affected by BCI, reproductive decisions (clutch size, laying date and percentage second broods) are not affected by the BCI levels in the following winter.

Perdeck A.C., M.E. Visser & J . H . v an Balen 2000, Ardea 88(1) : 99-106