| Summary: | The influence of human disturbance on the nesting success of the Common eider (Sonateria mollissima) was investigated in the Mingan Archipelago National Park Reserve during summers of 1994 to 19%. Six colonies were used, four located on open islets and two on forested islands. Three experimental treatments were used that entailed varying the frequency and time occurrence of visits in incubation. Every years, each treatment was applied to several eider colonies differing in the density of nesting gulls on the same island. During visits, incubating females were flushed and nest fate recorded. Several variables also were measured during visits describing female quality (assessed by clutch size and laying date) and nest site quality (assessed by nest concealment, distance to shore, vegetation height, and density of nesting gulls on the island). Results showed that frequency of visits did not affect nesting success of eiders, mainly because most observed nest failures occurred after the first visit The time occurrence of the first visit during incubation was of primary importance; eider nests visited early were much more affected by our presence. The nesting success of eiders was related to the density of associated colonies of nesting gulls only when visits occurred during early incubation. Female eiders of greater reproductive quality were those most successful when disturbed regularly. These females were in greater proportion on islands associated with lower density of nesting gulls, i.e., on forested islands rather than on open islets. Within colonies, they were mainly found nesting near shore where nest concealment is sparse. Results showed that visits to eider colonies can greatly affect their reproductive output. Visits during early incubation on open islets had the greatest impact.
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