Do nest-departure and nest-return rhythms of bumblebees align with community-level flower visitation patterns?

Bumblebees (Bombus spp.) are key high-elevation pollinators, yet their nest-level activity patterns and their synchronization with community-level floral visitation remain poorly understood. In previous studies, bumblebees showed bimodal floral visitation patterns in wild high-elevation communities, with activity peaks in the morning and afternoon. However, it is unknown whether nest activity mirrors these patterns in the field. Understanding this relationship is important for linking colony-level behaviour with pollination dynamics in natural communities. In this study, we investigated the daily departure and return behaviour of a natural colony of Bombus lepidus in the Hengduan Mountains, southwestern China, and their coordination with floral visitation in the nearby communities during the 2019 flowering season. We also examined associations between environmental factors and nest activity across the day. During the observation period, overall activity declined over the season, whereas the proportion of returning bees carrying pollen increased. Daily nest departures and returns exhibited pronounced bimodal patterns that closely paralleled floral foraging activity, albeit with slight temporal offsets between the independently sampled activity distributions. Colony activity was associated with nest-entrance environmental conditions, showing unimodal (non-linear) responses: activity peaked at a nest entrance air temperature of approximately 24 °C and nest entrance relative humidity of 50–55%. Morning activity was relatively insensitive to environmental variation, whereas afternoon behaviour was more responsive to microclimatic fluctuations. Our results suggest that nest-entrance microclimate may be linked to daily activity patterns and their temporal alignment with floral resources at a local scale.