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A marked population of Monk Parakeets Myiopsitta monachus was studied for eight years in two localities in Cordoba, ArgentiFia. We made observations on behaviour and measured population density, natality, mortality, emigration and immigration. Monk Parakeets are resident year-round, with limited movements during winter. They nest solitarily or in colonies. The Monk Parakeet is unique among parrots in that it builds large enclosed stick nests, which are often integrated in a compound nest that may include several isolated chambers. Parakeets continue to roost within their nests throughout the whole year. Single nests often develop into large compound nests in which different breeding pairs occupy separate nesting chambers. Members of a compound nest may also include nonbreeding individuals occupying separate chambers. These parakeets do not defend an all-purpose territory, defense being restricted to the nests. Juveniles continue to roost in the parental nest during autumn and winter but leave permanently before the following breeding season. Dispersal distance from birth to breeding place is reduced, averaging 1.2 km. We observed a few cases of atlofeeding; helping in nest building, maintenance, and vigilance were observed in all birds occupying a compound nest. Breeding was delayed until at least two years of age. Annual survival of first year birds was estimated to be 61%, whereas adult survival was 81%. Our results suggest that the Monk Parakeet's breeding system and population dynamics show several characteristics of a communal breeder, a breeding system not previously recorded in parrots.