Nutritional condition is an important determinant of productivity and survival in caribou (Rangifer tarandus). We used samples of excreta (n  =  1,150) to estimate diet composition from microhistology and 2 isotopic proxies of protein status for 2 ecotypes of caribou in 4 herds in late winter (2006–2008). Isotopes of nitrogen (δ¹⁵N in parts per thousand [‰]) from excreta samples (urea, diet, and body N) were used to estimate indexes of protein status: the proportion of urea N derived from body N (p-UN) and the difference between the δ¹⁵N of the body and urinary urea (Δ_body-urea). We examined dietary and terrain characteristics, δ¹⁵N, p-UN, and Δ_body-urea by ecotype, herd, year, and foraging site. Multiple regression and an information-theoretic approach were used to evaluate correlates of protein status at each foraging site. The dietary and terrain characteristics of foraging sites did not vary by ecotype or herd (P > 0.108); diets were dominated by lichens (68% ± 14.1% SD). The δ¹⁵N of urea, diet, body N, p-UN, and Δ_body-urea varied among foraging sites within each herd (P < 0.001). Although highly variable, the δ¹⁵N of urinary urea was typically low (−4.68‰ ± 2.67‰ SD). Dietary N also had low δ¹⁵N (−4.18‰ ± 0.92‰ SD), whereas body N was generally heavier in ¹⁵N (2.20‰ ± 1.56‰ SD) than urinary urea or the diet. Both measures of protein status were similarly diverse between ecotypes and among herds, which limited their applicability to monitor protein status at the population level. Although we observed limitations to interpreting estimates of p-UN from highly vagile ungulates, the Δ_body-urea may prove to be a useful index of protein status at smaller spatial and temporal scales. Indeed, a portion of the observed variance (r²  =  0.26) in Δ_body-urea at each foraging site was explained by the proportion of shrubs in the winter diet. There remains potential in using δ¹⁵N in excreta as a noninvasive tool for evaluating protein status in northern ungulates; however, considerable analytical and sampling challenges remain for applying these isotopic approaches at large scales.