. The Canadian field-naturalist. Natural history. 604 The Canadian Field-Naturalist Vol. 116. kilometres Figure L Location of the 14 Glaucous-winged Gull breeding colonies surveyed in the south- em Strait of Georgia, British Columbia. nesting Glaucous-winged Gulls shifted such that 61 % of the nesting gulls were found on the largest breeding colony, Mandarte Island, in 1997 compared to 45% in 1986 (Table 1). This is the first reported decline in the number of nesting Glaucous-winged Gulls within the Strait of Georgia in almost four decades. The decline in the number of nesting pairs of Glaucou
. The Canadian field-naturalist. Natural history. 604 The Canadian Field-Naturalist Vol. 116. kilometres Figure L Location of the 14 Glaucous-winged Gull breeding colonies surveyed in the south- em Strait of Georgia, British Columbia. nesting Glaucous-winged Gulls shifted such that 61 % of the nesting gulls were found on the largest breeding colony, Mandarte Island, in 1997 compared to 45% in 1986 (Table 1). This is the first reported decline in the number of nesting Glaucous-winged Gulls within the Strait of Georgia in almost four decades. The decline in the number of nesting pairs of Glaucous-winged Gulls could be related to an increase in colony disturbances, and in particular increased disturbance by Bald Eagles. On Mandarte Island, Bald Eagles were observed flushing gulls on all days researchers were present on the island in 1997 (Table 2). In addition, the number of eagle dis- turbances of nesting gulls within each day, in 1997, ranged from 1 to 23 times, with up to six eagles on Mandarte Island at any one time. Colony distur- bances during the nesting period result in egg and chick predation by Northwestern Crows and other Glaucous-winged Gulls (Verbeek 1982). On Mandarte Island, over 50 depredated gull eggs were found in a single crow cache in both 1996 and 1997. In addition to the increased number of breeding Bald Eagles in the Strait of Georgia, more than twice the numbers of eagles were observed in 1987 when compared to 1974 (Vermeer et al. 1989). An increase in the observed number of eagles on or near Mandarte Island was not detected until the late 1970s and early 80s. Since that time, eagle observa- tions occurred almost every day during the gull breeding season. Bald Eagles have negatively impacted other colo- nial nesting species in the Strait of Georgia. Increased Bald Eagle predation resulted in decreased breeding productivity of Great Blue Herons (Ardea herodias fannini) (Butler 1997; Vennesland 2000) and disturbances by Bald Eagles at Double-crest
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