DIURNAL RHYTHM OF COLONY ATTENDANCE AND OPTIMAL CENSUS TIME FOR THE BLACK GUILLEMOT CEPPHUS-GRYLLE IN THE BALTIC SEA

Counting the adult birds at breeding colonies is the only useful metho d 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 d uring three summers at Valassaaret, Gulf of Bothnia. The attendance al ways peaked in the early morning, soon after sunrise, and then decline d sharply. Early morning counts should thus be preferred for monitorin g, but counts conducted at other times of day can be transformed by us e of correction factors to estimate the peak numbers. Counts made dire ctly from the breeding islands are to be recommended in preference to undisturbed counts from a distance. Similar diurnal attendance pattern s have been found in most study areas in Europe; in seasonal rhythm th ere seems to be more local variation. The biological significance of t he pronounced early diurnal attendance peak at Black Guillemot colonie s is discussed briefly.