We observed Mexican Jays (Aphelocoma ultramarina) and Northern Flickers (Colaptes auratus) from June 2000 to March 2001, recording interspecific associations and interactions. Flickers were seen with jays only once in summer, but they were observed together 62 times in winter, in mixed flocks of up to 20 jays and 12 flickers, while jays were alone only 4 times, and flickers were alone 31 times. Jays always initiated flock movement, and flickers followed. Flickers were more likely to forage when they accompanied jays than when they were alone. While jays searched under and around oaks for acorns and grasshoppers, flickers probed for subterranean ants. We found no evidence that flickers attempted to rob the jays' acorn caches, and we observed no agonistic encounters between the two species. Avian predators approached mixed flocks on six occasions, and in each case flickers responded to jay alarm calls by flying with the jays into oak foliage. All predation attempts were unsuccessful. We conclude that Northern Flickers most likely were associated with Mexican Jays in winter because of increased security from predation, and not because of shared resources. Received 13 February 2002, accepted 17 December 2002.