UK breeding bird community temperature index rose 0.015 °C per decade in 1994–2024, about one third of full tracking, but not more than with randomly shuffled species temperature indices

Question: did UK breeding bird communities change toward warm-climate species from 1994 to 2024, and is this change specific to the thermal affinity of the species? Approach: we used the UK Breeding Bird Survey (6634 random 1-km squares, 85,713 square-years, 97 common native species). We calculated a species temperature index (STI) from GBIF June–July records in European 1-degree cells and WorldClim March–August temperature, and a community temperature index (CTI) for each square and year. We compared the within-square CTI trend with the Met Office HadUK-Grid March–August warming. We decomposed the trend exactly into species contributions. We tested the trend against a null distribution from 10,000 shuffles of the STI values among species. Results: the CTI increased by 0.015 °C per decade (95% two-way bootstrap interval 0.011 to 0.018), which is 31% of the trend for full tracking of the 0.21 °C per decade warming (interval 16% to 116%; 16 versus 49 km per decade as northward shifts). Shuffled STI values gave a trend this large or larger in 21% of cases (one-sided p = 0.21; within migratory groups p = 0.25; p = 0.07 to 0.33 in 11 analysis variants). STI did not predict BTO 1995–2023 population change (Spearman 0.10). Increases of warm species and decreases of cold species both added to the CTI, but decreases of warm species cancelled much of it. Goldfinch alone supplied 40% of the net trend. Year-to-year CTI changes correlated weakly with temperature changes (r = 0.22 to 0.24), also not more than with shuffled STI values. Conclusion: the CTI trend is real under site and year resampling, but these data do not show that thermal affinity explains it. Limitations: the STI comes from opportunistic GBIF records; the permutation test assumes exchangeable species and detects only strong tracking (above about 64% of full tracking); the analysis covers one country and 31 years.

community temperature index · climatic debt · Breeding Bird Survey · United Kingdom · species temperature index · permutation test · GBIF · climate change · birds · thermophilization · HadUK-Grid · population trends