Individual variation in diurnal body temperature and foraging activity in overwintering black-capped chickadees...

Small birds in winter can mitigate energetic shortfalls via increases in foraging and/or via controlled reductions in metabolic rate and body temperature (torpor). The ability to both increase foraging and use torpor during the day could have profound implications for an individual's daily energy budget and overwinter survival. Trade-offs between foraging efficiency and daytime torpor use may exist but have not been explicitly investigated. Here, we investigated the presence of within- and among-individual correlations between daytime body temperature (T_b, a proxy for torpor use) and foraging in overwintering black-capped chickadees (Poecile atricapillus). Using temperature-sensing passive integrated transponder tags, we measured daytime T_b and foraging in 20 free-living chickadees over 49 days in a single winter (January-February). Chickadees generally exhibited T_b around normothermic levels with an average T_b during visits to the feeder of 41.7 °C, though T_b ranged between 25.0 and 44.9 °C. Chickadees exhibited moderately lower daytime T_b, shorter time intervals between successive feeder visits (IVI), and increased feeder visits as ambient temperature decreased. However, within individuals there was only evidence of a weak positive correlation between visit T_b and IVI, and no correlation between daily feeder visits and daily mean visit T_b. We found that visit T_b, daily mean visit T_b, and daily feeder visits were repeatable, while IVI was not. Sex did not explain a significant amount of variation in total daily feeder visits or daytime T_b, nor was there evidence of among-individual correlations between daily mean visit T_b and daily feeder visits. Our results suggests that chickadees may independently regulate foraging and diurnal T_b. Overall, our study provides insights into how small birds in winter can use multiple strategies to overcome energetic challenges. Future studies investigating diurnal torpor and its integration with other strategies are needed to further elucidate how small birds survive harsh winter conditions.