OPTIMAL CLUTCH SIZE IN HERONS - OBSERVATIONAL AND EXPERIMENTAL APPROACHES

During an 8-year study, the clutches most frequently laid by Night Her ons Nycticorax nycticorax and by Little Egrets Egretta garzetta were 1 -egg smaller than the most productive ones, and this contrasts with th e idea that natural selection maximises reproduction. The productivity of the largest clutches in these two herons seems to be proximately L imited by their inability to hatch successfully clutches above four an d six eggs, respectively. Major losses seem to involve hatching failur es due to the parents' inability to incubate clutches above a certain size, or to early chick mortality due to competitive aggression among siblings. Clutch size and fledgling Success were stable or slightly in creased during the season, an unusual trend that may be linked to a la te peak in food availability. Among the hypotheses put forward to expl ain the discrepancy between most frequent and most productive clutches in birds, the evidence for Night Herons and Little Egrets contrasts w ith the predictions of the ''predation'', the ''bad years'', and the ' 'insurance'' hypotheses. A clutch size modification experiment, the fi rst one accomplished on a ciconiiform species, was conducted for 3 yea rs, and its results contrasted with the predictions of the ''individua l optimum'' hypothesis. The limited information now available suggest that the ''incubation capacity'' and the ''adaptation in progress'' hy potheses may be applicable to these herons, who find particularly abun dant food resources in the rice fields of this study area.