| Summary: | Counting the adult birds at breeding colonies is the only useful method for population monitoring in the Black Guillemot. Before the counts can be compared, however, information is needed on the optimal census time . Diurnal and seasonal rhythms of colony attendance were studied during three summers at Valassaaret, Gulf of Bothnia. The attendance always peaked in the early morning, soon after sunrise, and then declined sharply. Early morning counts should thus be preferred for monitoring, but counts conducted at other times of day can be transformed by use of correction factors to estimate the peak numbers . Counts made directly from the breeding islands are to be recommended in preference to undisturbed counts from a distance. Similar diurnal attendance patterns have been found in most study areas in Europe; in seasonal rhythm there seems to be more local variation. The biological significance of the pronounced early diurnal attendance peak at Black Guillemot colonies is discussed briefly.
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