Food, reproductive success and multiple breeding in the Great Tit Parus major

We studied the reproductive success of facultatively double brooded Great T its Parus major in relation to (seasonal) variation in abundance of their m ain food supply: caterpillars in Oak Quercus robur. Data were collected in two mixed woods (Vlieland and Hoge Veluwe, from 1985-1996). The caterpillar food stock is characterised by a strong peak in the breeding season, and h eight and timing of this peak vary between years. In first broods, nestling survival, number of fledglings and fledging mass were highest at the time of the food peak and lower before and after this time. Clutch size, and suc cess of the first clutch, were positively related to the caterpillar densit y on individual territories. On the annual level, multiple breeding (define d as the proportion of pairs starling a second clutch after fledging young from the first brood) occurred at increasing frequency as the tits bred ear lier relative to the food peak, while there was no additional effect of abs olute laying date. As has previously been shown, also within years multiple breeding decreased strongly with time. Early in the season, family-flocks foraged in Oak, and later switched to Pine Pinus nigra, and we hypothesise that multiple breeding is more frequent in early breeding pairs, because go od feeding conditions for the family flock may reduce the effect of a secon d clutch on fitness of the first clutch. Time that the family-necks spent i n Oak was longer when breeding was earlier relative to the food peak, and t he incidence of multiple breeding increased with increasing time spent in O ak, which provides correlational support for this hypothesis. The actual am ount of food present did not affect the occurrence of multiple breeding, al though the inter-clutch interval decreased with increasing food supply. The natural patterns were confirmed with a supplementary feeding experiment, w hich increased nestling growth, and resulted in shorter inter-clutch interv als, but had no effect on the occurrence of multiple breeding. Since succes s of the first clutch increased as the birds bred closer to the caterpillar peak, while the probability of a second clutch decreased, a trade-off exis ts between the fitness of the first clutch and the residual reproductive va lue of the parents, which will affect the optimal time of breeding. This tr ade-off is likely to be less important in single-brooded populations, where individuals should perhaps simply breed close to the food peak. This contr ast is illustrated by the finding that the annual timing of breeding in our two multiple breeding populations was not related to the date of the cater pillar peak, which contrasted significantly with the positive relationship previously reported for two single-brooded populations (Oosterhout, Wytham) .