den Boer-Hazewinkel J. (1987) On the costs of reproduction: parental survival and production of second clutches in the Great Tit. ARDEA 75 (1): 99-110

In Great Tits a negative correlation of the production of a second clutch after a successful first brood and the survival of the parents could not be established. On the contrary, there seems to be a positive correlation between these two variables. The existence of this correlation in two habitats, a pine and a mixed wood, was established by analysing the data of 14 years using log-linear models. These analyses showed this correlation to be independent of habitat, age and sex of the parents and of significant correlations among these variables and between these variables and reproductive output or survival. Nor can this correlation be explained sufficiently by differences in survival and in production of second clutches between years. The same positive correlation between the production of a second clutch and survival of the parents was found in the data of two years in an optimal habitat, a deciduous wood. Since most second broods are produced by Great Tits in their most marginal habitats (with the highest annual mortality), combining data of different habitats may obscure this correlation and even turn it into a negative one. It is suggested that this may be an explanation for the contradictory results obtained within this species. To demonstrate the existence of costs of breeding an experiment was made to reduce the costs of breeding for part of the breeding population by the provision of plenty of extra insect food during the first brood period. If breeding involves costs in terms of a reduction in the survival chances of the parents, one expects parents. to compensate for a reduction in the costs of breeding by producing more and/or better offspring in the same year and/or to show a better survival until the next breeding season. In the habitat (considered to be optimal for Great Tits) and in the period (the first brood period) in which the two-year experiment was done none of these effects could be demonstrated. It is suggested that in such an optimal habitat the period before the incubation of the eggs of the first brood may be a more critical period.