I examined enemy recognition in Field Sparrows (Spizella pusilla) using models of a brood parasite (Brown-headed Cowbird, Molothrus ater), an avian nest predator (Blue Jay, Cyanocitta cristata), and a nonthreatening control (Fox Sparrow, Passerina iliaca) during two periods of the nesting cycle: the first three days of incubation and when chicks were 2–3 d old. As predicted, Field Sparrows responded more frequently to the predator model than to the control model during both periods, and gave stronger responses to the predator model (as measured by the number of “chip” calls) during the nestling period than during incubation. Field Sparrows also gave stronger responses to the cowbird model than to the control model during incubation. However, responses to the cowbird model during incubation were not stronger than the responses during the nestling period; this result does not support the hypotheses that Field Sparrows recognize cowbirds either as brood parasites or as potential nest predators. If nests were tested during the laying stage, when they are most vulnerable to parasitism (but prone to abandonment due to disturbance), it is possible that Field Sparrows would respond most aggressively to cowbirds during that period.