Clutch Size, Timing of Laying, and Reproductive Success in a Colony of Great Blue Herons and Great Egrets

Abstract

AI3STRACT.--During a 13-yr period mean Great Blue Heron (Ardea herodias) clutch size at a central California colony ranged between 2.72 and 3.35 eggs, with an overall mean of 3.16. Mean Great Egret (Casmerodius albus) clutch size ranged from 2.70 to 3.07 eggs, with an overall mean of 2.87. Annual differences in clutch size were weakly significant in both species, and clutch size in both species declined slightly with relative as well as absolute time of breeding. The overall mean number of fledglings produced from heron nests was 1.45, with annual means ranging from 0.89 to 2.38. Year, clutch size, and relative time of breeding had significant effects on heron fledging success. The mean number of fledged egrets/nest was 0.90, ranging from 0.03 to 2.04 annually; only annual differences accounted for a significant proportion of the variance in fledging success. In both species, the most frequent clutch size was 3 but the most productive size was 4. Overall, 46.8% of the heron eggs and 33.0% of the egret eggs fledged young. The herons nested in smaller numbers and earlier than the egrets. Annual variability in colony mean clutch-initiation dates was less for herons than for the egrets. Egret clutch and brood sizes were smaller before the 1972 ban on DDT and related chemicals than after. Laying dates before and after the ban did not differ significantly. A similar comparison for the herons showed no significant differences associated with the ban on DDT. The herons lost more young to starvation than to predation, and the egrets displayed the opposite trend. Mortality of the young was greatest for the herons in the second week of life and greatest for the egrets in the third; survival for both species was high after 5 weeks. Except for a drop in the number of breeding pairs of egrets after a year of heavy predation, fluctuations in population levels were not correlated with reproductive biology. Received 9 December 1983, accepted 7 July 1984.