We examined the responses of resident flocks of white-throated sparrows to unfamiliar individuals and vocalizations throughout the winter, while also examining circulating levels of the stress hormone corticosterone, to see if they paralleled behavioral changes. Concurrently, we validated identification of sparrow plumage morphs with genotype assays to determine reliability of field identification. Accuracy of field identification of plumage morphs was 68.8%. Baseline corticosterone did not differ among sampling periods, suggesting these birds did not experience prolonged chronic stress throughout the winter. However, white-throated sparrows responded more aggressively to study skins and playback of conspecific calls and songs in November than in January and March. These results suggest that agonistic displays may be more important for defending winter territories and establishing dominance status in early winter.