| Summary: | Known for its exceptional biodiversity, Vigur Island (Westfjords, Iceland) welcomes several bird colonies each year. This 2 km long, 400 m wide island also hosts an eiderdown farm and touristic visits from late June to September. Hence, the aim of the 2021 summer research campaign was to estimate the population of different bird species, and to establish their abundance in order to create a valuable database that could be used for further research and monitoring. The bird census was conducted both on land and sea, as far as permitted, with the use of binoculars. We focused on the main bird species found on the island: black guillemots, Northern fulmars, great cormorants, both Herring and Lesser black-backed gulls, Eurasian oystercatchers, and Arctic terns. Atlantic puffins were very common on the island, but no count was made for this species. The Arctic tern population was estimated by dividing the land into square units and subsequently counting nests using the line-over-line method. More precisely, the Arctic tern colony population was estimated using a transect line method (20 x 30m units). Due to the hatching of the eggs, the survey had to be stopped and only covered 60% of the colony area. Other species were counted by two observers following the coast of the island 5 times, with a maximum of two days apart. Gulls were counted together to avoid likely misidentifications. Potential differences among observers were addressed through a Wilcoxon-Mann-Whitney test for all bird species. The results show that in summer 2021, Vigur potentially hosted 1092±246 black guillemots, 28±8 oystercatchers, 19±8 cormorants, 120±34 fulmars, and 58±20 gulls. We counted 440 occupied Arctic tern nests, leading to an estimation of 880 breeding adults. The guillemot population has increased up to threefold over previous or local estimates (Náttrufristofnun Islands, 2021b). Comparison for the other species was impossible, as no counts were previously conducted. This report also details difficulties and specificities linked to the ...
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