Urbanization creates new ecological landscapes that may alter the behavior of animals occupying them. Northern cardinals (Cardinalis cardinalis) inhabit both urban and rural habitats in northeastern Oklahoma, where food availability, predator type, and plant phenology can differ. Relative to birds in rural habitats, cardinals in urban environments formed smaller nonbreeding groups during winter, began singing earlier in the spring and responded more strongly to temperature, and occupied territories at higher densities. These results are in accord with the hypothesized greater food availability (provided by bird feeders), larger numbers of mammalian predators (domestic cats), and earlier availability of nesting sites (through earlier plant leaf-out because of the heat island effect) in urban areas. Urbanization has led to relatively dramatic changes in northern cardinal social behavior and singing phenology.