Wildlife surveys are essential for understanding population sizes and trends and allow for accurate assessments of their conservation status and the development of effective management measures. This article proposes and describes a census method for the Black-headed Gull Chroicocephalus ridibundus on the Basque coast. Additionally, it analyses its repeatability and compares the results obtained with those of the traditional January census of wintering waterbirds. The protocol assumes that most individuals leave estuaries late in the day to spend the night at sea. Therefore, surveys are conducted at the estuary mouth at one hour before sunset. The average number of birds counted per estuary ranged from 50 birds at Ondarroa to 1,240 birds at the Abra. The model (function 'rpt') estimated a very high repeatability (0.80, SE = 0.10). It was observed that on average 65% of the gulls went to sea during the last 15 minutes before sunset, while less than 10% did so in the 15-minute interval between one hour and 45 minutes before sunset. For the whole Basque coast, 3,213 Black-headed Gulls were counted in the winter of 2020/21, 2,274 in 2022/23 (although in the latter winter, the Oria River estuary, which in 2020/21 contributed 157 gulls, was not counted) and 2,725 in 2023/24; this difference was not significant (taking into account the Oria River estuary). The protocol proposed in this article allows for a more realistic estimate of the size of the wintering population of Black-headed Gulls on the Basque coast than the traditional January waterbird census, which is not specific. However, it is intended only as a complementary method that is not designed to replace the wintering waterbird census, which is highly valuable due to its long time series. Furthermore, the method can be extrapolated to other wintering areas of this species and, possibly, to other species with similar behaviour.

Arizaga, J., Aldalur, A., Alzaga, A., Amenabar, J., Carazo, Ó., Carriel, A., Delgado, S., Galarza, A., García, N., Pacheco, J. L., Ruiz, J., Salvatierra, E., Santamaría, D., Valdés, G., Valiente, A., Zorrozua, N., Zubiaur, J., 2025. Protocolo de censo de gaviota reidora Chroicocephalus ridibundus en invierno en la costa vasca y comparativa con el censo internacional de aves acuáticas. Rev. Cat. Ornitol. 41, 21-31.

Ornitología