COSTS AND RATES OF EGG FORMATION IN RUDDY DUCKS

We analyzed composition of rapidly developing ovarian follicles and eggs of Ruddy Ducks nesting in southern Manitoba in 1988. For ducks with at least one ovulated follicle, there was a significant correlation between amount of fat in the largest developing follicle and time of day that ducks were shot. If ovulations occur, on average, 24 hr apart, then this correlation indicates a high degree of synchrony of ovulation in this population. We used regression analysis to estimate that 95% of ovulations occurred between early evening (18:17 CST) and mid morning (08:45 CST). These results support observations by Siegfried (1976) and Tome (1984) that the interval between eggs is 24 hr in wild Ruddy Ducks. We never found more than six rapidly developing follicles per female, and given that eggs are laid daily, duration of rapid follicle growth is 5-6 days. This conflicts with an estimate of 11 days for follicle 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 against basal metabolic rate (BMR, 208 kj), is among the highest (280% of BMR) recorded for waterfowl. However, Ruddy Ducks that lay seven eggs, i.e., the modal clutch size, would incur maximum costs for only one day.