Swedish bird community temperature index rose 0.06 °C per decade on 714 routes in 1996–2025, about half the long-term warming rate, but did not follow route warming rates or annual temperature

Question: do breeding bird communities in Sweden and Norway change their composition as fast as the climate warms, and is the change larger where the local warming is larger? Data: Swedish Bird Survey fixed routes (714 routes, 11,994 route-years, 1996-2025) and the Norwegian TOV-E scheme (449 routes, 5,648 route-years, 2006-2025). We calculated species temperature indices (STI) for 494 species from the EBCC Atlas of European Breeding Birds (1961-1990 April-July GHCN-CAMS temperature). Local temperatures came from E-OBS v33.0e. We calculated the abundance-weighted community temperature index (CTI), within-route trends and the spatial CTI-temperature slope, and tested tracking at three scales. The main 95% intervals come from a two-way bootstrap (100-km route blocks x 3-year year blocks). Results: In Sweden the CTI increased by 0.060 °C per decade (0.034 to 0.081). The increase was positive in all 11 sensitivity analyses. In Norway it increased by 0.031 (-0.017 to 0.081), and the Norwegian increase depended on non-passerines. Across routes the CTI changes by 0.34 °C per °C of local climate. Against the 1981-2025 warming (0.36 °C per decade), Swedish communities followed 0.48 (0.25 to 0.84) of the change that full tracking predicts. With the survey-year warming as the reference, the ratio is not constrained (-1.6 to 3.6) when year-to-year temperature variation is included, and it changes from 0.32 to 0.91 with the season and climate data source. On 652 Swedish routes, the slope of the route CTI trend on the local warming rate was -0.23 (-0.42 to 0.06). Full local tracking predicts 0.34, or about 0.2 after attenuation for the measured reliability of the warming pattern. Annual CTI values did not follow local temperature anomalies: two-way fixed-effects slopes were -0.008 (-0.033 to 0.016) in Sweden and -0.005 (-0.022 to 0.011) in Norway. Increases of warm-dwelling species and decreases of cold-dwelling species contributed almost equally to the Swedish increase. Interpretation: community composition moved slowly toward warm-dwelling species, at about half the rate of the long-term warming, and this movement was not a local response to temperature. Limitations: the STI is a fixed atlas-based species value; the spatial slope can include habitat gradients; route locations are approximate; the route test has power only for Sweden 1996-2025; the results are observational associations.

community temperature index · climatic debt · breeding birds · Sweden · Norway · climate change · species temperature index · thermophilization · E-OBS · EBBA · bootstrap · monitoring