BROOD-MANIPULATION EXPERIMENTS .2. A COST OF REPRODUCTION IN BLUE TITS (PARUS-CAERULEUS)

1. Potential reproductive costs in blue tit (Parus caeruleus) parents were assessed by experimentally manipulating brood-size and measuring survival and fecundity costs between years. 2. Using logistic models, female survival was found to be independent of both clutch-size and manipulation, as well as brood-size. This was true when data from all 3 years were combined, and for each year separately. 3. Male survival was independent of clutch-size when data from all 3 years were combined, but declined significantly with increasing manipulation. Adding brood-size to a model of male survival only weakly (P < 0.1) explained the data. The negative association between male survival and manipulation was apparently due to a single year: when data from each of the 3 years were analysed separately only 1986 showed a significant negative effect of manipulation and brood-size on male survival. 4. Measures of both male and female fecundity in year n + 1 were not found to be negatively influenced by the size of the clutch, extent of the manipulation or size of the brood reared in year n. 5. Data given by Nur (1984a, 1988b) pertaining to a cost of reproduction in a population of blue tits where brood-size was also experimentally manipulated were reanalysed using logistic models. In only 1 of 3 years was female survival significantly negatively associated with the size of brood reared. In males, there was a significant positive association between male survival and brood-size in 1 year, but were independent of each other in another. 6. These analyses of both my own and N. Nur's data suggest that the evidence for a cost of reproduction in blue tits is weak. Possible reasons for the discrepancies between mine and N. Nur's conclusions are discussed.