ABSTRACT. Many migratory songbirds switch from a primarily insectivorous diet during the breeding season to either a mixed diet or fruit diet during the non-breeding season. However, for species with mixed diets, arthropods may be superior food items because of their higher protein content and easier digestibility. We tested this hypothesis by analyzing the diet and body condition of omnivorous Wood Thrushes (Hylocichla mustelina) at a non-breeding site in tropical forest in Belize, Central America. We used analysis of stable isotopes d 15 N and d 13 C in the blood to measure diet. Our objective was to determine if a higher dietary proportion of arthropods relative to fruit (i.e., higher d 15 N and d 13 C) was associated with better body condition. We also examined the possible effect of age, sex, and habitat type on Wood Thrush diets, as well as any changes in diet through the overwintering period. We used a hierarchical Bayesian mixing model (MixSIAR) to estimate the proportion of different prey items in the diet of overwintering Wood Thrushes overall, in each habitat type, and over time during the non-breeding period. From January to April, we found a significant decline in d 15 N in forest habitats, whereas d 15 N increased in scrub habitat. There was no significant seasonal change in d 13 C. Birds with higher d 15 N or d 13 C values were not in better body condition. Females in dry-scrub habitat consumed more fruit than males, but this did not affect body condition. Mixing model results indicated that most Wood Thrushes at our study sites consumed primarily arthropods, even during the driest times of the non-breeding season and in the driest habitat. Overall, our results suggest that the diet of Wood Thrushes varies with habitat and during the overwintering period, but diet alone was not a predictor of body condition. Wood Thrushes, and possibly other omnivorous migratory songbirds, are apparently flexibly able to meet their wintering and pre-migration nutritional demands with a variety of diets.RESUMEN. La proporci on de artr opodos versus frutas en la dieta influencia la condici on f ısica durante el invierno en un ave canora omn ıvora? Muchas aves canoras migratorias cambian de una dieta principalmente insect ıvora durante la temporada reproductiva a una dieta o bien mixta o bien frug ıvora durante la temporada no reproductiva. A un as ı, para especies con dietas mixtas, los artr opodos pueden ser items alimentarios superiores debidos a su mayor contenido prot eico y f acil digesti on. Pusimos a prueba esta hip otesis analizando la dieta y la condici on f ısica del omn ıvoro Zorzal del Bosque (Hylocichla mustelina) en un sitio no reproductivo en el bosque tropical de Belize, Am erica Central. Utilizamos an alisis de is otopos estables d 15 N y d 13 C en la sangre a fin de medir dieta. Nuestro objetivo fue determinar si una proporci on m as alta de artropodos en relaci on a fruta en la dieta (i.e., d15 N y d 13 C m as alto) est a asociada con una mejor condici on f ısica. Examinamos tambi en el posible ef...