COSTS AND RATES OF EGG FORMATION IN RUDDY DUCKS

We analyzed composition of rapidly developing ovarian follicles and eg gs of Ruddy Ducks nesting in southern Manitoba in 1988. For ducks with at least one ovulated follicle, there was a significant correlation b etween amount of fat in the largest developing follicle and time of da y that ducks were shot. If ovulations occur, on average, 24 hr apart, then this correlation indicates a high degree of synchrony of ovulatio n in this population. We used regression analysis to estimate that 95% of ovulations occurred between early evening (18:17 CST) and mid morn ing (08:45 CST). These results support observations by Siegfried (1976 ) and Tome (1984) that the interval between eggs is 24 hr in wild Rudd y Ducks. We never found more than six rapidly developing follicles per female, and given that eggs are laid daily, duration of rapid follicl e growth is 5-6 days. This conflicts with an estimate of 11 days for f ollicle maturation made by Gray (1980) who counted pairs of light and dark rings in egg yolks. We explain this incongruity by proposing that egg-laying Ruddy Ducks may show a diphasic, and perhaps crepuscular, foraging schedule whereby two sets of rings are deposited/24 hr (Dobbs et al. 1976). Maximum daily cost (584 kJ) of egg production, scaled a gainst basal metabolic rate (BMR, 208 kj), is among the highest (280% of BMR) recorded for waterfowl. However, Ruddy Ducks that lay seven eg gs, i.e., the modal clutch size, would incur maximum costs for only on e day.