Macronutrient (protein, lipids, and carbohydrates) assimilation efficiencies of wild birds have rarely been studied, but they may be particularly important for estimating the optimal diet. We assessed experimentally the energy and macronutrient assimilation efficiencies of captive Snowy Plover (Charadrius alexandrinus) feeding on the polychaete ragworm Nereis diversicolor, a major natural prey for migratory shorebirds (Charadrii) along intertidal habitats worldwide. Overall, Snowy Plover consumed 8.64 ± 1.54 (SD) g of dry ragworms per day, and the apparent metabolizable energy was 136.59 ± 27.69 kJ·day-1 (3.5-4.0 times the basal metabolic rate). Assimilation efficiencies were 80.04 ± 2.04%, 93.48 ± 1.08%, 84.75 ± 2.01%, and 89.23 ± 2.61% for energy, proteins, lipids, and carbohydrates, respectively. Macronutrient assimilation efficiencies were similar during early winter and premigration and between male and female Snowy Plover. Relatively high protein assimilation by Snowy Plover may be associated with their specialized high-protein diet. © The American Ornithologists' Union, 2008.