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Abstract Historical data suggest that many bee species have declined in body size. Larger‐bodied bees with narrow phenological and dietary breadth are most prone to declines in body size over time. This may be especially true in solitary, desert‐adapted species that are vulnerable to climate change such asCentris pallida(Hymenoptera: Apidae). In addition, body size changes in species with size‐linked behaviours could threaten the prevalence of certain behavioural phenotypes long‐term.C. pallidasolitary bees are found in the Sonoran Desert. Males use alternative reproductive tactics (ARTs) and are dimorphic in both morphology and behaviour.C. pallidamale body size has been studied since the 1970s in the same population. The authors collected body size data in 2022 and combined it with published records from 1974–2022. The authors find a persistent decline in the mean head width of patrolling males, and shifts towards smaller body sizes in the populations of males found foraging and hovering. Both morphs declined in average body size, and the proportion of large‐morph males in the population decreased by 8%. Mating males did not decline in mean body size over the last five decades. The authors discuss hypotheses related to the decline inC. pallidamale head width. Finally, the authors advocate forC. pallidaas an excellent study system for understanding the stability of ARTs with size‐linked behavioural phenotypes.