The capacity for quick and easy assessment of an animal’s physiological condition has broad applications for fundamental and applied field ecologists. Many point of care devices—such as personal glucose metres readily available at pharmacies—have been validated to measure metabolites in non-human animals, and use in field studies is increasing. With this boom in interest, field experiments are critical for understanding the mechanisms driving individual variation in circulating metabolites in wild, free-living populations. We used point of care devices and a food supplementation experiment on pre-breeding black-legged kittiwakes (Rissa tridactyla) to examine the effects of food availability, sex, and date on four blood metabolites: glucose, ketones, cholesterol, and triglycerides. Food supplementation altered blood metabolite concentrations. These differences were more pronounced in pre-breeding females than males, possibly due to differences in energetic demands leading up to egg-laying. However, the most notable changes were shifts in blood metabolite concentrations as the pre-breeding season progressed; glucose, ketones, and triglycerides increased over time, while cholesterol decreased over time. These results indicate that regulation of metabolite concentrations could be a mechanism for maintaining energy balance under different diets. We also offer recommendations for using point of care devices in field settings, and discuss implications for conservation physiology.