| Summary: | In many seabird colonies along the coast of Norway, the increased population of white-tailed eagle Haliaeetus albicilla and the growing industry of birdwatching tourism are causing a dilemma for management decisions. Thus, this study aimed to examine the indirect effect of white-tailed eagles and tourists on the breeding success of the endangered seabird species, black legged kittiwake Rissa tridactyla in the bird cliffs at Hornøya Island. By investigating kittiwakes’ escape response (the number of empty/abandoned nests) provoked by eagle disturbance, as an indirect top-down effect on egg survival by facilitating nest predation by crows, ravens and/or larger gulls, we examined this indirect effect by asking three questions: (1) Is the escape response among kittiwakes stronger when white-tailed eagle is present? (2) Is the escape response independent of nest location or are the nests in the periphery of the study plot more frequently abandoned (testing the diluting hypothesis – safety in numbers)? (3) Is there a negative relationship between disturbance and egg survival? The data used to answer these questions, were collected by Reconyx Hyper Fire time-laps cameras deployed in two test plots (plot A and B), where plot A was located in a non-disturbed sheltered area of the cliff and plot B was installed next to a tourist trail. In each plot a sample set of n=30 active kittiwake nests was made. Number of eggs were counted, in addition to number of white tailed eagles in the area. The number of tourists were provided by the local business of birdwatching tourism with a total number of 220 tourists visiting the study area in May and June. By fitting generalised linear mixed-effect models the results showed that the escape response was stronger when white-tailed eagle was present, with a much stronger response in plot B. Moreover, it was likely a diluting effect in plot B, as the nests in the periphery were more frequently abandoned, and the distance between abandoned nests were much smaller than in plot A. Nevertheless, a trend found in the data indicates that high frequency of escape response may have a risk effect of limiting egg survival, thus affecting the breeding success of black-legged kittiwake. Lastly, we discuss potential factors causing the differences found in escape response between and within the study plots and the application of time-laps cameras as a non-intrusive tool in long-term monitoring of interactions between kittiwakes, white-tailed eagles, nest predators and tourists in a seabird colony.
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