Urban habitat has become more and more important for birds because of rapid urban development and reduction of natural areas, and the responses of birds to season in urban habitats is important to research in urban avian ecology. We examined whether bird communities varied with season in terms of community attributes, composition of species and species groups, and nestedness in highly urbanized Hong Kong, which is positioned on one of the main migratory routes, and thus has great conservation value for birds. We studied 30 urban parks, and had 3,815 observations of 31 species in the breeding season (May–Aug 2010), compared to 3,972 observations of 53 species in the nonbreeding season (Nov 2010–Feb 2011). Eurasian Tree Sparrows (Passer montanus), Red-whiskered Bulbuls (Pycnonotus jocosus), and Japanese White-eyes (Zosterops japonicus) were the most abundant species in both seasons. Bird species richness and diversity in the nonbreeding season were significantly higher than in the breeding season. Migrants accounted for 5.0% of total abundance and 28.3% of richness in the nonbreeding season. Compared to the breeding communities, the avian communities included more insectivores (4.2%), insectivore-frugivores (4.8%) and herbivores (10.5%), but fewer granivores (8.4%) and species that feed on the ground (10.7%) appeared in the nonbreeding season. Moreover, multi-response permutation procedures analyses showed the composition of species and species groups differed significantly between the two seasons. Bird communities were highly nested in Hong Kong, but the degree of nestedness was close for the two seasons. We found there are obvious seasonal patterns in the bird community of Hong Kong, and an understanding of this can provide basic, essential information that park managers can employ to conserve a more diverse ecosystem in Hong Kong and even in other cities.