Frequency and distribution of behaviour of red-necked grebes breeding colonially and in classic territories

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.

Comparative study of retinal oil droplets in grebes and coots

A study of the relative proportions of coloured retinal oil droplets in the five Canadian grebe species and the American Coot was undertaken to evaluate the possibility of an ecological rationale to the pattern of variation in these proportions among the species. Statistical analysis suggests that there may be significant differences in these proportions among the six species studied, and that some species show significant heterogeneity among the nine sectors into which the retinas were divided. There was no significant association of oil droplet proportions in either Western or Red-necked grebes with two different lake types. Significantly greater mean total droplet densities were found in the Pied-billed Grebe and the American Coot than the other four grebes; the lowest droplet densities were shown by Western and Eared grebes. The finding of significant within-retina (among-sector) variation, and high levels of individual variation in some species throws into question reported values for other species where single samples have been taken from one or a few individuals.