Breeding biology of Great Blue Herons and Common Egrets in Central California


Pratt, H. M.

Publication Date:
Jan 1970

Information available in the ornithological literature about breeding biology of the Great Blue Heron (Ardea herodius) and the Common Egret (Casmerodius albus) is fragmentary at best. Breeding behavior preceding and during pair formation of the Great Blue Heron has been described by Bent (1926), Cottrille and Cottrille (1958), and Meyerriecks (1960). Miller (1943) comments on some aspects of breeding biology of the Great Blue Heron in the Philadelphia region but his data suffer from lack of quantification. In fact, the recurrent themes in the most complete summary to date of information about this species (Palmer 1962) are “quantitative data lacking,” and “more information needed.” Studies of the Common Egret are even more limited than those of the Great Blue Heron. Little has been added to knowledge of its reproduction since Bent (1926) wrote its life history. Gersbacher (1939) reports on clutch size and breeding behavior in Tennessee. Meyerriecks (in Palmer 1962) summarizes current information and Teal (1965) reports on nesting success in Georgia. Quantitative data on incubation period, nest attentiveness, and rates of feeding the young have been non-existent. The number of young fledged in any given heronry, population fluctuations from year to year, and factors affecting nesting success are largely unknown. This study has produced basic information about reproduction in Great Blue Herons and Common Egrets nesting at Audubon Canyon Ranch, California, in 1967 and 1963, and quantitative data about behavior at the nest of Great Blue Herons. Audubon Canyon Ranch is a sanctuary situated on the central California coast just north of San Francisco. Here the wooded western slope of Bolinas Ridge descends to Bolinas Lagoon, a shallow, tidal estuary which serves as a major feeding ground for both herons and egrets. They build their nests in the tops and upper branches of the coast redwoods (Sequoia sempervirens) that grow to a height of 80-100 ft in the bottom and on the north-facing slope of a steep and narrow canyon. From an observation point constructed on the south-facing slope of the canyon, observers can look over the tops of the nesting trees and study the breeding birds. Nests are about 100-200 yards from the overlook, and the birds appear to be undisturbed by the presence of observers. All observations were taken from this overlook. None of the other Ardeidae has been observed nesting at Audubon Canyon Ranch to date, although a few Snowy Egrets (Leucophoyx thulu) perched briefly in the heronry in both years of the study. Black-crowned Night Herons (Nycticorax nycticorux) and Snowy Egrets commonly feed in the lagoon, and Green Herons (Butorides virescens) are occasionally seen there also.

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Pratt, H. M. 1970. Breeding biology of Great Blue Herons and Common Egrets in Central California. Condor 72: 407-416.