THE INFLUENCE OF INVESTIGATOR DISTURBANCE ON THE BREEDING SUCCESS OF RING-BILLED GULLS (LARUS-DELAWARENSIS)

Increased losses of eggs and chicks resulting from human intrusion (investigator or other) into seabird colonies have been well documented. From 1989 to 1991, we studied the effects of investigator disturbance on the breeding success of individual pairs of Ring-billed Gulls (Larus delawarensis) nesting at two colonies near Port Colborne, Ontario, Canada. The insular colony was on an artificial breakwall, associated with the Welland Ship Canal, approximately 1 km off the north shore of Lake Erie. The mainland colony was adjacent to the Canal approximately 1 km east of the breakwall. Clutch size, hatching success/pair and fledging success/pair were measured for pairs nesting in study plots subjected to various levels of investigator disturbance. The disturbance level of each study plot varied with respect to visitation frequency and investigator activities performed on each visit. Chicks were considered fledged at 21 days old. Visits into the study areas before and during the egg-laying period had no significant effect on nesting density or clutch-size distribution. Similarly, visits during the incubation phase had no effect on the hatching success of study pairs. Finally, while we did not enter the study areas during the brooding period, we established that visits during hatching had no effect on the hatching or fledging success of study pairs. The abundance of available cover provided shelter in which mobile chicks crouched during disturbances (investigator or other).