Wintering raptors in lowland farmland of north-western Italy: a second distance sampling survey twenty years later

Abstract Diurnal raptors and the great grey shrike ( Lanius excubitor ) wintering in the lowland farmland of northwestern Italy were surveyed again after 20 years, during the winters of 2020/2021 and 2021/2022. Data were collected using roadside car transects, and perpendicular distances of birds from the transect lines were recorded to estimate wintering raptor densities. Ten species (9 raptors) were observed, but only two species, the common buzzard ( Buteo buteo ) and the common kestrel ( Falco tinnunculus ), were recorded frequently enough to allow reliable density estimates. Extensive rice fields in the north-eastern part of the study area supported high densities of common buzzards, with estimates ranging between 1 and 2 individuals per km². Although occurring at lower densities (~ 0.5 individuals per km²), the common kestrel showed a broader habitat tolerance and appeared less dependent on specific agricultural landscapes. The estimated wintering population of common buzzards in the lowland farmland of the Piemonte region, an area covering approximately 5,700 km², was slightly higher than that recorded 20 years earlier, ranging between 4,000 and 5,000 individuals. For the common kestrel, the estimated wintering population in the same area was around 2,500 individuals (1,800–3,500). However, the total wintering populations of both species in the whole region were certainly much larger, because they are also widespread during winter in hilly and mountainous areas. Overall, the results highlighted the importance of rice fields for wintering common buzzards, suggested stability in the raptor community, and advocated for broader adoption of distance sampling methods for effective large-scale bird surveys.