We studied the avifauna in a subtropical humid forest around Hotel El Tirol, Department of Itapúa, Paraguay. The study site is part of the Atlantic Forest biome, a critically endangered biodiversity hot spot. We recorded 205 species of birds in 45 families and provide information on their annual cycles and natural history from collected specimens and field observations. Increased knowledge will contribute to efforts to maintain and manage the bird species of this endangered biome. The annual cycle of the avifauna at El Tirol closely resembled that of birds at north temperate and subtropical latitudes. Birds entering reproductive phases were likely responding to increasing photoperiod and rainfall. In general, breeding was initiated in mid-August and began to decline in late November. Molt rose sharply in early December and continued into April, overlapping with breeding in 16 species. In some species, males replaced juvenal plumage with an immature plumage easily distinguishable from the adult form, which they acquired after several years. Most males in immature plumage bred. For most species skull pneumatization in young of the year was completed during their first year. In several species, however, the pneumatization process extended into year 2, and in other taxa, complete pneumatization requires multiple years or never occurs. The birds at El Tirol were predominantly insectivorous. About a quarter of the specimens in 134 species had noticeable body fat. In general, fat deposition peaked in June, with lesser peaks from February to mid-March and in late August and September. Twenty-seven of the species that we recorded were migratory. We compared the species of birds recorded at El Tirol with those recorded from San Rafael National Park, a large tract of Atlantic Forest farther north in Paraguay with a much more extensive avifauna. Some of the species we failed to record were likely overlooked. However, El Tirol is at the southwestern limit of the Atlantic Forest habitat; absences may reflect faunal attenuation as species reach the limits of their ecological ranges. The lower numbers may also indicate that diversity in forest patches is reduced. Regardless, such patches will play an important role in maintaining the Atlantic Forest avifauna.