Museum bird specimens from North America became lighter within locations in 1901–2024, but the decrease did not follow local temperature and was about 18 times the spatial-slope prediction

Question: Does the body mass of birds change in time as Bergmann's rule predicts from space? We did a test with 141,722 sexed, non-young, dated and georeferenced bird specimens with a recorded mass from 29 museum collections in the USA, Canada and Mexico (1901–2024, 1,615 species). We took the masses from the Darwin Core dynamicProperties of VertNet and GBIF-registered archives. We joined each specimen to CRU TS 4.09 cell temperature (0.5 degree) and, in the contiguous USA, to nClimGrid. Within species, after we removed the effects of sex, season and collection, mass decreased by 0.37% per degree C between cells (95% CI 0.33–0.42). In the same cell, mass decreased by 0.72% per decade (95% CI 0.50–0.86; 58,169 specimens, 402 species, 784 cells). The cells became warmer by 0.105 degree C per decade, so the spatial slope predicts only 0.039% per decade. The observed decrease is 18 times the prediction (95% CI 11–28) and 9 to 32 times in 13 sensitivity analyses (May–July only, no salvage text, fat class controlled, one collection, nClimGrid, sexes, recorded adults, open-licence records, 1976–2024). The decrease did not follow the temperature history: mass did not decrease when the cells became warmer from the 1910s to the 1930s, it decreased by about 2.5% from the 1970s to the 1980s without warming, and it did not decrease further after the 2000s while the cells became warmer. With a time trend or decade effects in the model, the 10-year temperature slope was near zero (+0.05 ± 0.46 and +0.47 ± 0.48% per degree C), and the trend stayed. Without a trend term, the same slope was −3.2% per degree C, which is an artefact of the common trend. Warmer 12-month periods gave 0.41 ± 0.12% lower mass, near the spatial slope. Recorded salvage, tissue and fat data did not explain the decrease. Limitations: collection and weighing methods changed in ways that the records mostly do not show; we did not analyse structural size; MVZ gives 38% of specimens. Conclusion: within-location mass decrease in museum birds is real in the data but local warming through a Bergmann-type effect does not explain it, so specimen mass trends alone are not evidence of a response to warming.

Bergmann's rule · body size · body mass · climate change · museum specimens · birds · space-for-time substitution · VertNet · Arctos · GBIF · CRU TS · nClimGrid · North America · shrinking body size