Nutritional support is a primary therapy administered to oiled animals during responses to oil spills, but data informing nutritional decision-making during events are limited. In this study, 44 common murres (Uria aalge) and 6 Western grebes (Aechmophorus occidentalis), naturally oiled by oceanic seeps off the coast of Ventura and Santa Barbara Counties, CA, USA, were assigned to 1 of 4 groups fed diets with varying levels (6.8% [no added oil], 11%, and 20%) and types (salmon, corn) of oil added to a partially purified basal diet. Birds used in the study ranged from extremely emaciated to thin body condition (62%–80% wild bird mean body mass). Acid-insoluble ash was used as an indigestible dietary marker to quantify nitrogen retention, apparent nitrogen digestibility, nitrogen-corrected apparent metabolizable energy, energy digestibility, fat retention, fat digestibility, and estimated fat excretion. Fat excretion is important in these species because once birds have been cleaned they are at risk of plumage recontamination from excreted fat during care. Lower fat diets resulted in lower fat excretion but higher nitrogen retention, higher apparent nitrogen digestibility, and higher apparent metabolizable energy. Decreases in nitrogen retention were significantly related to increases in fat excretion. Regardless of diet, energy digestibility significantly declined with declines in body mass, suggesting severity of emaciation reduced a birds' ability to extract energy from food. Energy digestibility was highest in the 11% (low) salmon oil diet; hence, this diet had the highest effective energy content despite a lower gross kcal/kg diet. Diets fed during oil spills historically have had high fat concentrations to provide maximum caloric support. Results of this study suggest that lower fat diets may be more efficacious for nutritionally depleted seabirds. This study provides valuable data to guide clinical decision making regarding nutritional support during oil spills and other mass stranding events.