Double-crested Cormorants of the Great Lakes: egg-laying parameters, reproductive failure, and contaminant residues in eggs, Lake Huron 1972–1973

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Abstract
During the 1960s Double-crested Cormorants breeding on Lake Huron experienced a decline in numbers and an elevation in body and egg contaminant burdens. In 1972 and 1973 the Canadian Wildlife Service visited their colonies to collect demographic data and eggs for residue analysis. In 1972, colonies were small. They suffered high egg breakage and loss (95%) and nearly total reproductive failure (0.06–0.11 young per nest). Eggshells were 23.9% thinner than normal. Clutch size (2.0 ± 1.2 eggs) was low but egg production (4.1 ± 1.9 eggs) appeared normal. Subsequent clutches appeared in 45% of the nests. Levels of DDE (14.5 ± 5.6 ppm, wet weight) and PCBs (23.8 ± 9.6 ppm) in the eggs were higher than in other cormorant populations in Canada. Levels of DDT, DDD, dieldrin, heptachlor epoxide, hexachlorobenzene, and mercury were less than 1 ppm. Organochlorine levels in eggs from Georgian Bay were similar to those from the North Channel, but mercury levels were higher. Colony size continued to decline in 1973, although reproductive success improved (0.3 young per nest). These data may be used to assess subsequent changes in the status of Lake Huron cormorants which unpublished observations indicate are now recovering rapidly.