. The Canadian field-naturalist. 274 The Canadian Field-Naturalist Vol. to the Canadian Wildlife Service under contract CWS 72/73-005). Methods The distribution of Herring Gull colonies was investigated early in the breeding season. Six col- onies were selected for estimating breeding suc- cess and these were visited approximately once weekly when weather permitted. Seven other col- onies were visited and samples of eggs were re- moved but no estimate of breeding success was made. During each visit to the colonies where breeding success was being estimated, the number of nests and of eggs were
. The Canadian field-naturalist. 274 The Canadian Field-Naturalist Vol. to the Canadian Wildlife Service under contract CWS 72/73-005). Methods The distribution of Herring Gull colonies was investigated early in the breeding season. Six col- onies were selected for estimating breeding suc- cess and these were visited approximately once weekly when weather permitted. Seven other col- onies were visited and samples of eggs were re- moved but no estimate of breeding success was made. During each visit to the colonies where breeding success was being estimated, the number of nests and of eggs were determined. A small sample of eggs was removed from these colonies to determine eggshell thickness, measured by a dial micrometer, and to determine the content of or- ganochlorine residues by the method outlined by Reynolds (1969). Chicks which hatched on the islands were identified by banding. Eggs which did not hatch but which were fully incubated were collected and the stage of development at death estimated. The distribution of the 13 colonies which were studied is shown in Figure 1. There was one colony in the St. Lawrence River on Black Ant Island; colonies studied in eastern Lake Ontario were lo- cated on Pigeon Island (See Figure 2), southwest of Wolfe Island; on two islands in the Kingston Basin, Snake Island and Brother Island; on Scotch Bonnet Island and Nicholson Island; and Presqu'ile Provincial Park. The only colony vis- ited in western Lake Ontario was on Muggs Island near Toronto. Colonies were located at Port Col- borne and Port Maitland (Mohawk Island) in east- em Lake Erie and on Big Chick Island in the west basin. Two colonies were visited in the Georgian 0 Brien Islands Brother slands. Black Ant Island Snake Island Pigeon Island Chick islands STATUTE MILES Figure L Distribution of Herring Gull Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of
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