We studied seasonal variation in bird assemblages in a Chaco subtropical semiarid forest, Santiago del Estero Province, Argentina. We carried out seven samplings during both breeding and non breeding periods, in order to assess the resident status and the trophic guild of each species. During the last four samplings we recorded bird populations using 30 fixed-radius points, in order to estimate bird density, species density, evenness, and species diversity. We found that: (1) the bird assemblage had 96 species (67 residents, 19 summer migrants, 1 winter migrant, 5 of doubtful residency and 4 species whose resident status was unknown); (2) higher values of species density, evenness and species diversity were found during breeding periods; and (3) higher community attributes were followed by higher densities of 4 trophic guilds: nectarivores, frugivores and short and long-flight insectivores during the breeding periods. Changes in guild densities were probably associated with the arrival of summer migrants and to seasonal fluctuations in food resources, which in turn promoted seasonal changes in species density and diversity. Different from observations in other avian assemblages of the Chaco, no temporal variations were observed in the terrestrial seedeaters guild. This could be due to the fact that our study site forests are located in a landscape matrix with agricultural fields and grasslands that may offer year-round seed supply. The Chaco subtropical semiarid forest seems to be an important breeding habitat for many migrant species from both Neotropical and Nearctic regions.