Data from: Birkhead TR, Montgomerie RD 2023 Census counts of Common Murres adjusted for timing of breeding are more accurate than counts based on calendar dates. Ornithological Applications 125:duad036. https://doi.org/10.1093/ornithapp/duad036

Climate change has resulted in marked advancement in the breeding phenology of some bird species. Changes in the timing of breeding can have important biological effects, for example causing mismatches from the food supply. Using data and simulations, we show that the two-week advance in the timing of breeding in common guillemots Uria aalge between 1973 and 2020, on Skomer Island, Wales, also has serious implications for the timing of census counts. As a result, because censuses have been conducted during the same fixed calendar period each year, the size of the breeding population has been underestimated. We recommend that, when appropriate information on the timing of breeding is available, censuses of breeding seabirds be made relative to the median egg-laying date rather than on specific calendar dates as is usually the case. 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 guillemots at other colonies, and to other species in other geographic regions.