Ground-nesting is usual among the about fifty species of dabbling ducks (genus Anas), but a few southern hemisphere species prefer arboreal sites. One such arboreal-nesting species, the South American Speckled Teal (Anas flavirostris flavirostris), is believed to have evolved from a ground-nesting ancestor, represented today by the closely related Green-winged Teal (A. carolinensis) of North America. We studied the breeding biology and behavior of Speckled Teal in Buenos Aires province, Argentina, in an attempt to identify adaptations associated with arboreal nesting. In this region, Speckled Teal prefer to nest in cavities in the stick nests of Monk Parakeets (Myiopsitta monachus). Inter- and intraspecific competition for cavities was intense. Parakeets remained near the nest site year round, aggressively defending occupied cavities and forcing teal to compete with one another for possession of abandoned cavities. Male Speckled Teal helped their mates to obtain and hold nest sites by defending sites against other pairs. Males also accompanied broods and assisted in parental care. Although courtship occurs year round and extrapair courtship by males is common, competition for mates and the need for male assistance in acquiring nest sites limits the opportunities for polygyny in this population. Site fidelity of nesting females was very high; 19 of 22 returned the following year. The prevalence of courtship on land and inclusion of Point display in the repertoire are interpreted as behavioral adaptations associated with arboreal nesting.