Tongue length predicts phenological patterns in coexisting Bombus species

Phenology, the timing of life cycle events, often varies among species and determines the environmental and floral resources available for foraging pollinators. In some taxa, ecological traits are important predictors of interspecific variation in phenology. Bombus species differ in life history traits such as body size, tongue length, and nesting behavior. These traits are known to reflect strategies related to foraging efficiency, thermal tolerance, and nesting microclimate. Here, we test whether these traits also explain differences in phenological timing among Bombus species within a subalpine ecosystem. Using standardized surveys conducted weekly from May to October 2023, we documented the seasonal activity of nine Bombus species. Phenological timing (emergence, peak, and end dates) was estimated using quantiles derived from species activity distributions, and associations between traits and phenology were evaluated using generalized linear models. Only tongue length significantly predicted phenological timing, where longer-tongued species emerged and peaked earlier in the season than medium-tongued species. Body size and nesting location showed no significant trends, though larger-bodied and above-ground nesting species tended to emerge earlier. We hypothesize that this difference reflects longstanding patterns of floral resource availability, with deeper-tubed flowers earlier in the season and shallower flowers later in the season. These findings suggest the possibility of subtle phenological mismatches between Bombus species and flower morphology in changing environments.