Breeding bird data from the Côa River Basin (Portugal): twenty-six years of change in a Mediterranean landscape in transition

Breeding bird data from the Côa River Basin (Portugal): twenty-six years of change in a Mediterranean landscape in transition This data package brings together standardized breeding-bird transect data collected in the Côa River Basin, north-eastern Portugal, over twenty-six years. It combines data from two editions of the Portuguese Breeding Bird Atlas with an independent directed re-survey, together with the vector layers needed to reproduce their spatial framework. It accompanies a data paper submitted to the Biodiversity Data Journal. Survey periods T1 (1999–2005): 1,004 thirty-minute transects in 251 2×2 km squares, from the 2nd Portuguese Breeding Bird Atlas, provided by the Instituto da Conservação da Natureza e das Florestas (ICNF). Species presence per transect. Survey dates: 5 April 1999 to 29 June 2005. T2 (2015–2021): 153 thirty-minute transects in 153 2×2 km squares, from the 3rd Portuguese Breeding Bird Atlas, provided by the Sociedade Portuguesa para o Estudo das Aves (SPEA). Counts of individuals. Survey dates: 29 April 2015 to 24 June 2021. T3 (2023–2025): 276 thirty-minute transects in 233 2×2 km squares, collected by a single observer (A. E. Monteiro) following the same atlas protocol. Counts of individuals. Survey dates: 10 April 2023 to 13 June 2025. The three periods are published as separate datasets and are not merged, as they derive from distinct survey programmes and, in the case of T2, from a different reference grid. Spatial framework T1 and T3 use the UTM-based 2×2 km grid of the 2nd Atlas (713 squares over the basin). T2 uses the 2×2 km grid derived from the ETRS89 / LAEA Europe 10×10 km grid adopted by the 3rd Atlas (731 squares overlapping the basin). The two grids differ in origin, alignment and cell footprint and are distributed as separate layers. All coordinates and vector layers are in ETRS89 / Portugal TM06 (EPSG:3763), in metres. Coordinates in the bird-count tables are 2×2 km square centroids. Taxonomy Scientific names follow the IOC World Bird List v14.2. The package covers 143 species. Records not identified to species level and domestic or infraspecific forms are retained in the spreadsheets but excluded from the species count. Files (each ZIP contains the data and an individual README) SM1: bird records 1999–2005 (.xlsx and .csv) SM2: bird counts 2015–2021 (.xlsx and .csv) SM3: bird counts 2023–2025 (.xlsx and .csv) SM4: UTM-based 2×2 km grid (shapefile) SM5: Côa River Basin boundary, approx. 2,524 km² (shapefile) SM6: LAEA-based 2×2 km grid (shapefile) SM7: GPS tracks of a subset of the 2023–2025 transects (shapefile) Access conditions The historical atlas data were provided by ICNF and SPEA for scientific use within the PhD project "Measuring rewilding's ecological effects on bird communities and its impact on nature conservation in the Côa valley". ICNF confirmed in writing (5 November 2025) that the 2nd Atlas data may be used in scientific publications provided that they are aggregated at 2×2 km and that no sensitive locations are disclosed. All bird records are aggregated at 2×2 km, and no nest coordinates or other sensitive breeding-site locations are included. Commercial use of any part of this package requires prior written consent from the authors and, for SM1 and SM2, from ICNF and SPEA respectively.