At early breeding stages, male woodpeckers invest heavily in nest construction and defense, but parental contributions to brood defense among Picidae are not well known. We studied the Northern Flicker (Colaptes auratus) to determine whether sex, age, brood size, body size, or body condition influenced defense behavior. When presented with a model predator (red squirrel, Tamiasciurus hudsonicus) during the brood-rearing period, parents exhibited a range of behaviors, such as blocking the nest hole, diving at the model, and striking the model; however, defense scores did not differ between males and females aged 1, 2, or 3 years old. Although we predicted that defense level would be positively correlated with brood size, we found no such relationship. Adult body size and condition also were not related to defense intensity. We conclude that the sexes may exhibit similar levels of defense because they have similar apparent annual survival rates and males are only slightly larger than females. If flickers optimize clutch size according to the number of offspring they can rear, then there may be no relationship between defense and brood size.