BROOD-MANIPULATION EXPERIMENTS .1. THE NUMBER OF OFFSPRING SURVIVING PER NEST IN BLUE TITS (PARUS-CAERULEUS)

1. Brood-size was experimentally altered in blue tits (Parus caeruleus) over three breeding seasons. The survival of offspring from unmanipulated and manipulated nests was determined. In addition, previously published work by Nur (1981 et seq.) was reanalysed. 2. Specifically, the individual optimization hypothesis was evaluated: females should lay that size of clutch which maximizes the survival of their offspring. Parents with young either added to, or removed from, their nests should have lower numbers of surviving offspring that subsequently breed than control nests. 3. My own data showed that parents rearing a brood equal in number to their own original clutch-size maximized the number of surviving young: adding or removing young did not increase the number of recruits per nest. The extent to which a brood was either larger or smaller than the original clutch-size was termed 'manipulation'. A log-linear model of offspring survival showed that year effects (entered as a factor), clutch-size, manipulation and its square all significantly reduced the amount of variance (deviance) present in the data. 4. An inverse quadratic function best described the relationship between offspring survival and manipulation for each clutch-size: the turning points of these fitted lines were not significantly different from zero. 5. Reanalysis of N. Nur's data also provided support for the individual optimization hypothesis: the number of young surviving per nest was maximized when manipulation equalled zero. 6. These results are consistent with the hypothesis that females lay that size of clutch which maximizes their recruitment potential: rearing a larger or smaller brood than that which they set out to raise does not increase the fitness of individual parents.