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The White-crested Elaenia (Elaenia albiceps chilensis) is a Neotropical austral migrant that breeds in Patagonian forests and overwinter in South America tropics. We investigated White-crested Elaenia demography and the environmental conditions the specie is subjected at the northwest of Patagonia as a first attempt to elucidate its population dynamic. We studied the species abundance, individuals' body condition, survival, productivity of young and site fidelity according to changes in rainfall patterns and food supply, during three breeding seasons (2009 to 2012). We color banded 361 adult and juvenile White-crested Elaenias. During the first studied season, precipitations were remarkably above the mean and ripe fruit supply was lower than in the other seasons, whereas the abundance of arthropods remained similar along the study. The species abundance decreased 50% across the study, while adult apparent survival rate, the number of breeding females, young productivity and site fidelity were 50% lower in the first season in comparison to the following seasons. Females presented lower body condition at arrival in the first two studied periods. Site fidelity diminished after unfavorable breeding season (i.e. low young productivity and fruit supply), especially for males, although they were more faithful than females and had a slightly higher apparent survival rate. Our results suggest that White-crested Elaenia demography is affected by changes in its main food resource supply (fleshy fruits) during the breeding season, indicating that this part of the annual cycle could be fundamental for its population dynamic.