A migratory herbivore responds to rapid changes in food supply: diet and body condition of an arctic caribou herd at onset of green-up.

Annual migration of ungulates is often correlated with regional and seasonal plant growth, but are smaller scale movements of individuals related to fitness? We investigated the response of wild caribou to variation in plant phenology. We used faecal transects to evaluate spatial differences in diet in one growing season after the onset of spring plant growth had occurred (green-up). We captured 110 females (1-15 years) from the Teshekpuk Caribou herd over three years (14-28 June 2022-2024). We measured diet and corresponding indices of body condition of captured caribou at various stages of green-up. Plant growth (normalized differentiation vegetation index) at capture locations was greatest in 2023 when captures followed the onset of green-up. We sampled faeces and blood to measure concentrations of albumin and urea N in serum, and to measure δ15N and δ13C to track N and C from diet. Faecal concentrations of N (crude protein) indicated that diet quality was highest in 2023 (20.3%) after onset of green-up when caribou were closest to the coast and lowest in 2022 (5.6%) before onset of green-up when caribou were furthest inland. Serum albumin (4.4 g/dL) and urea N (9.9 mmol/L) concentrations were greatest in 2023, which was consistent with high N supplies after onset of green-up. Faecal δ15N values were lowest in 2022 and highest in 2024, which suggests diets dominated by lichens and graminoids, respectively. In 2023, even though some samples were from later into green-up, intermediate values for δ15N and low values for δ13C in blood and faeces were consistent with browsing near the coast. Changes in diet that can influence gain of body protein and thus the growth and reproductive potential of females were related to locations of caribou in the 14d window of green-up.