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Timing and frequency of census counts of cliff-nesting auks
Lloyd, C. (1975) British Birds 68(12):507513
Regular counts of the numbers of Razorbills Alca torda and Guillemots Uria aalge in a colony throughout the breeding season give valuable information on the variation in attendance. Counts of this kind were made at six of the colonies covered by the annual population monitoring scheme. Most counts were made between 08.00 and 12.00 hours to avoid variation due to diurnal attendance patterns. Counts were least variable during the nestling period and only slightly more variable at other times during the month of June. Providing this month coincides with the nestling period for most birds in the colony, census counts can most conveniently be carried out in June. An error of 17% to 46% in Razorbills and 13% to 26% in Guillemots must be expected if only a single count is made at a colony; with five or ten counts this error is considerably reduced (5% to 17% for Razorbills and 4% to 8% for Guillemots with ten counts). The accuracy of counts made during Operation Seafarer in 1969-70 and the annual census at a sample of colonies since are assessed.

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