. The Canadian field-naturalist. 258 The Canadian Field-Naturalist Vol. 95. ^ metres Figure I. Relative position of nests in trees occupied by both herons and cormorants at Lake Helena. In 1976, cormorants occupied nests 2,5,6, 8,9, 10, 1 l,and 12, and herons occupied nests 1 and 3. Nest 11 was occupied by herons early in the season but was later taken over by cormorants. fully fledged young in 1976, but no active nests of either species were found here in 1977 or 1978. Townsend colony. Both herons and cormorants shared nest trees in the Townsend colony during all three years of this study. In
. The Canadian field-naturalist. 258 The Canadian Field-Naturalist Vol. 95. ^ metres Figure I. Relative position of nests in trees occupied by both herons and cormorants at Lake Helena. In 1976, cormorants occupied nests 2,5,6, 8,9, 10, 1 l,and 12, and herons occupied nests 1 and 3. Nest 11 was occupied by herons early in the season but was later taken over by cormorants. fully fledged young in 1976, but no active nests of either species were found here in 1977 or 1978. Townsend colony. Both herons and cormorants shared nest trees in the Townsend colony during all three years of this study. In 1978, at least 43 heron and 24 cormorant nests were active. Nests were located near the tops of Black Cottonwoods (Populus tricho- carpa) 14-19 m high. M ost cormorants nested in dead or dying trees with little foliage near the edge of the colony, although some nested with herons in densely leafed-out trees near the center of the colony. As at Lake Helena, no apparent horizontal or vertical strat- ification of nests was observed. One tree, which sup- ported two heron and five cormorant nests in 1976, supported nine active cormorant nests and no herons in 1978. Some trees not used in 1976 had nests of one or both species in 1978. At least seven trees supported both heron and cormorant nests in 1978. Nesting Chronology and Behavior Lake Helena colony. During the three years of the study, herons first began using nests in late March; by the first week of April, most herons had arrived. The earliest arrivals selected old nests in the center of the main colony. In 1976, pairs were first seen on 22 March. The peak of display, courtship, and territory establishment was around 27 March - 10 April. Incu- bation extended through late April and early May, and fledging occurred from mid-June until mid-July. Herons started nesting later in that portion of the colony later occupied by cormorants, perhaps because the open, nearly dead trees offered less security. In 1976, the first two cormo
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