Historical data suggest that human climate and landscape modifications have caused many bee species to decline in body size. Larger-bodied bee species with narrow phenological and dietary breadth are most prone to declines in body size over time, potentially due to temperature increases during development or declines in resource allocation during development. This may be especially true in solitary, desert-adapted species that are vulnerable to climate change – such as Centris pallida (Hymenoptera: Apidae). In addition, body size changes in species with size-linked behaviors could threaten the prevalence of certain behavioral phenotypes long-term. C. pallida (Hymenoptera: Apidae) are large-bodied solitary bees with narrow phenological and dietary breadth found in the Sonoran Desert. Males use alternative reproductive tactics (ARTs) and are dimorphic in both morphology and behavior. Large-morph males are fixed on a patrolling and fighting-based mate location strategy, while small-morph males primarily use a sight-based hovering strategy. C. pallida male body size has been studied since the 1970s in the same population, allowing for comparison of population-level male morphological data associated with unique behaviors. We collected data on male body size in 2022 and combined it with published records to analyze body size trends from 1974-2022. We find a persistent decline in the mean head width of patrolling males, as well as distributional shifts towards smaller body sizes in the populations of males found foraging and hovering. However, mating males did not experience a decline in mean body size over time. We discuss several hypotheses related the decline in average C. pallida male head width. Finally, we advocate for C. pallida as a particularly good study system for understanding the stability of ART systems with size-linked behavioral phenotypes.