Great Blue Heron (Ardea herodias) parents alternate in performing incubation and brooding duties and ordinarily, if undisturbed, attend their nests continuously from the onset of incubation until the chicks are 21 to 28 days old (Pratt 1970). In the spring of 1976, however, my co-workers and I found that during unusually hot weather, some herons left eggs and chicks less than three weeks old unattended for short periods in order to drink. The Audubon Canyon Ranch heronry, located about 5.8 km N of Stinson Beach, Marin County, California, has been described previously (Pratt 1970). The nests are in the tops of Coast Redwoods (Sequoia sempervirens) and can be seen easily from a hillside above nest level. Each nest was plotted and numbered on a panorama of the heronry and written records were kept of nest progress. Bolinas Lagoon, a shallow estuary at the mouth of the canyon where the colony is located, is the herons’ major feeding ground.