Climate change has resulted in a marked advancement in the breeding phenology of many bird species. Since the timing of many monitoring programs 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.
How to Cite
Birkhead, T. R., and R. D. Montgomerie (2023). Census counts of Common Murres adjusted for timing of breeding are more accurate than counts based on calendar dates. Ornithological Applications 125:duad036.
LAY SUMMARY
Census counts of birds are often conducted at the time they most accurately reflect the size of the breeding population.
For many temperate zone species, breeding seasons are reasonably predictable, so census counts of specific populations have usually been conducted on the same calendar dates each year.
But, as climate change has resulted in the advance of many breeding seasons, calendar dates may no longer identify the most accurate census period.
Using data from a long-term study of Common Murres on Skomer Island, Wales, we show that using fixed calendar dates for censusing has underestimated the size of the breeding population, as the breeding season varies from year to year and has advanced by 2 weeks, on average, since 1990.
We advocate conducting censuses relative to the median egg-laying date each year to minimize the errors associated with counts on fixed calendar dates.
El cambio climático ha resultado en un adelanto marcado en la fenología reproductiva de muchas especies de aves. Dado que el ajuste temporal de muchos programas de monitoreo se basa en fechas del calendario, los cambios temporales de las estaciones reproductivas de las aves pueden dar lugar a un desajuste con el período de censo. Utilizando datos de un estudio poblacional a largo plazo de Uria aalge en la Isla Skomer, Gales, junto con simulaciones, mostramos que el adelanto de dos semanas en el momento de la reproducción de U. aalge entre 1973 y 2020 tiene graves implicaciones para el ajuste temporal de los conteos censales. Mostramos que debido a que los censos se han realizado tradicionalmente durante el mismo período fijo del calendario cada año, el tamaño de la población reproductora ha sido subestimado. Recomendamos que los censos de aves marinas reproductoras se realicen en relación con la fecha mediana de puesta de huevos en lugar de fechas específicas del calendario. Dado que el cambio climático ha resultado en un adelanto generalizado en el momento de las temporadas reproductivas de las aves en el hemisferio norte, nuestros resultados pueden ser relevantes para Uria aalge en otras colonias y para otras especies de aves en todo el mundo.
