An acute handling challenge mobilizes glucose and free fatty acids in mountain Dark-eyed Juncos (Junco hyemalis oreganus)

Resource mobilization and reallocation is a major endpoint of the physiological response to acute challenges. However, energy metabolites have not been the focus of most field studies. Here, we characterized the glycemic and free fatty acid (FFA) response to an acute-handling challenge in a breeding, free-living, population of Mountain Dark-eyed Juncos ( Junco hyemalis oreganus) in Mono County, CA. Juncos ( n = 12) were trapped in seed-baited potter traps at Tioga Pass Meadow, coinciding with territory establishment and early nesting period. Blood samples were collected at 0, 15, and 30 min post-capture, and analysis showed Juncos mobilized both glucose and FFA in response to an acute handling challenge. Blood glucose levels were significantly higher than baseline at 15 min (42.6% increase) and 30 min post-capture (66.7% increase). FFA levels were also significantly higher than baseline at 15 min (29.2% increase) and 30 min post-capture (22% increase). We also measured morphometrics (e.g., wing chord length, tarsus length, etc.), fat, and mass for each individual to investigate the effects of body size and fat storage on energy mobilization using linear models. We found no significant effects of body size or fat and mass on FFA mobilization. However, body size had a significant effect on glucose mobilization, with larger individuals displaying a higher percent increase in glucose from baseline to 15 min. Additionally, we found an effect of sex on glucose mobilization, with males showing a higher percent increase in glucose from baseline to 15 min. These results suggest that energy mobilization capacity is influenced by body size and male Juncos may increase energy mobilization relative to females during the breeding season.