Human disturbance and nesting success of Common Eiders: interaction between visitors and gulls

Common Eider colonies often are subjected to human visitors, such as down collectors, recreationists and researchers. However, the effects of frequency and timing of disturbance, and the abundance of nearby avian predators on eider nesting success have been studied only partly. We used three experimental treatments and six eider colonies over 3 years (1993-1995) to test the effects of these factors on eider nesting success, while controlling results for associated-gull nest density. Treatments consisted of (1) high frequency visits (once every 3 days) starting early in the incubation period (HFE), (2) low frequency visits (once every 15 days) starting early in the incubation period (LIFE), and (3) high frequency visits starting late in the incubation period (HFL). Analysis of covariance indicated that both disturbance treatments and associated gull nest. density had a significant effect on eider nesting success probability. Nesting success probabilities were similar for eiders under HFE and LFE treatments (means=0.317+/-0.166 [SE] and 0.434+/-0.172 respectively), indicating that changes in frequency of visits had little impact on nesting success. In contrast, timing of visits had a major influence on nesting success, as the HFL treatment resulted in a significant higher nesting success probability (mean=0.981+/-0.191) than the HFE treatment. Most nest failures occurred after the first visit in all treatments, although the impact of the first visit was lowest in the HFL treatment. Researchers and wildlife managers should visit eider colonies as late as possible, and avoid visiting colonies associated with high densities of eider egg predators. (C) 2002 Elsevier Science Ltd. All rights reserved.