Census counts of Common Murres adjusted for timing of breeding are more accurate than counts based on calendar dates

Climate change has resulted in a marked advancement in the breeding phenology of many bird species. Since the timing of many monitoring programmes is based on calendar dates, changes in the timing of birds’ breeding seasons may result in a mismatch with the census period. Using data from a long-term population study of Common Murres (Uria aalge; Common Guillemots in Europe) on Skomer Island, Wales, together with simulations, we show that the 2-week advance in the timing of breeding in Common Murres between 1973 and 2020 has serious implications for the timing of census counts. We show that because censuses have traditionally been conducted during the same fixed calendar period each year, the size of the breeding population has been underestimated. We recommend that censuses of breeding seabirds be made relative to the median egg-laying date rather than on specific calendar dates. Since climate change has resulted in a widespread advance in the timing of birds’ breeding seasons in the northern hemisphere, our results may be relevant to Common Murres at other colonies, and to other bird species worldwide.