Repeated Winter Occurrence and Roost Use by Western House Martins Delichon urbicum at an Inland Thermal Refuge in Northern Iberia

The Western House Martin Delichon urbicum is a trans-Saharan migrant whose Iberian winter distribution is concentrated mainly in southern and coastal areas. We document repeated winter occurrence and roost use at the Almirante Sarmento Rodrigues bridge over the River Douro (Duero) at Barca d’Alva, north-eastern Portugal, approximately 200 km from the Atlantic coast. Six opportunistic fixed-point surveys were conducted between 4 November 2021 and 1 January 2022: two in late autumn (4 and 9 November) and four in mid-winter (6, 21 and 29 December 2021 and 1 January 2022). Birds were recorded on all visits. The evidence of wintering rests on the four mid-winter visits, which included a maximum simultaneous count of 20 individuals (6 December 2021) and birds entering old nests beneath the bridge in the late afternoon (29 December 2021 and 1 January 2022). The absence of marked birds precludes confirmation that the same individuals remained throughout the period. According to atlas data and eBird maps, this is, to our knowledge, the northernmost non-coastal locality in Iberia with repeated winter records of the species. This single-site observation cannot establish a causal relationship with climate change, and continued standardised monitoring is required to determine whether such wintering recurs.