Passerines lack functional salt glands and to a large extent avoid feeding on marine invertebrates. An exception is Cinclodes nigrofumosus. Previous studies reported that the contents of its gastrointestinal tract had a lower osmolality than seawater suggesting that birds were supplementing their marine invertebrate diet with terrestrial invertebrates and fresh water at certain periods of the year. We report seasonal changes in the diet of C. nigrofumosus at two contrasting coastal localities in Central Chile and document the potential correlation between weather and feeding habits with osmoregulatory features in this species. We expected that C. nigrofumosus would increase its use of osmotically challenging marine dietary sources when fresh water is available. Animals were collected at El Quisco and Los Vilos from central Chile during the mist cool winter and during the hot and dry summer. Immediatly after capture several physiological and morphological variables were determined at the field. We used stable isotope analysis as a proxy variable to measure the relative contribution of marine and terrestrial sources to the diet of Cinclodes nigrofumosus. Our results support the notion that osmoregulatory factors constraint dietary selection: in both localities birds consumed more marine prey when environmental conditions were less stressing, i.e., when lower environment temperatures, higher humidity and more fresh water was available. Our results also indicate that osmoregulatory physiology does not respond in a similar fashion in both seasons and suggest that osmoregulatory physiology of birds is affected by environmental variables in a complex fashion