Over the two-year period, 1997 to 1998, nearly half of a population of over 200 red-necked grebes (Podiceps grisegena) breeding within a large lake (2,537 ha) in central Minnesota, chose to opportunistically nest in concentrated groups on large floating mats of cattail (Typha spp.). In this study, we examined how members of this typically territorial grebe species coped with living in a more social environment. The behaviour of pairs that nested close to conspecifics differed from that of solitary pairs throughout the nesting season. Colonial pairs tended to be less likely than solitary birds to leave their nest vacant and unguarded from competitors or predators. During the pre-nesting stage, colonial females and males spent more of their time in aggression than did solitary-nesting birds. However, after a pair established their nest and egg-laying began, female aggression declined dramatically in both colonial and solitary pairs. During the egg-laying period, females spent significantly more time on the nest than did males, while males were more active than females in nest defense. Throughout the incubation period, colonial males remained closer to the nest and their females than did solitary males. In general, birds nesting in close proximity exhibited a compressed use of the space surrounding their investments in the platform, mate, eggs, and even young, all of which may have become more vulnerable to competitors and predators in areas of higher aggregation. Examining how opportunistically colonial species adapt to breeding in close aggregations may provide insight into the early evolution of colonial nesting birds.