Nesting ecology of a population of red-necked grebes in Northwestern Ontario

Abstract

One of the largest, documented breeding populations of Red-necked Grebes (Podiceps grisegena holboelli) in the world was studied at Whitefish Lake, Ontario in 1993 and 1994. Whitefish Lake (WFL) represents a unique area compared to other study sites that show mostly solitary nesting Red-necked Grebes or a few pairs/lake. The population of nesting Red-necked Grebes at WFL is large for this species. It is also exceptionally dense (mean 1.01 pair per hectare) for this territorial species and could be considered a semi-colonial situation. The mean number of pairs nesting on the lake for 1993 and 1994 was 49 (range 59-39). The objective of the study was to expand on the limited information available on the Red-necked Grebe and to acquire data on nest and nest-site characteristics, egg measurements, clutch size, egg laying period, incubation period, hatching success, and young produced. Census results for 1993 show that peak nesting occurred on 21 June with 59 nests with eggs. Total number of eggs reached a maximum for 1993 at n = 202 for 21 June. Total nests with eggs peaked n = 39 on 22 June, 1994 while total eggs (n = 135) peaked on 30 June, 1994. The population is strongly associated with uncultivated wild rice (Zizania palustris) stands in shallow bays of the lake. Shallow, uniform water depth, and the high productivity of Whitefish Lake provide abundant food and vegetation for grebe breeding activities. Eighty-five percent of 121 nests in 1994 were constructed primarily of wild rice, the most abundant emergent species in the study area. One hundred and six of the 121 (88%) of the nests at Whitefish Lake were floating nests attached to the lake substrate by a column of sub-surface vegetation and detritus. Nest-site selection in Red-necked Grebes is influenced by underwater characteristics such as water depth, availability of nest material and anchors for the nest. Early evidence of future plant emergence, (future) shelter from wind and waves and protection/concealment from predators and some form of anchorage (debris, sticks or logs) evident only from underwater searches. A factorial ANOVA revealed significant differences between nest and non-nest sites for depth and vegetation density. Water depth at nests (57.4 ± 35.3 cm, n = 180) was significantly shallower than non-nest sites (86.9 ± 27.9 cm, n = 120). Overall vegetation density was higher for nest sites than non-nest sites. Mean distance for nearest neighbour for 148 nests at Whitefish Lake was 27.2 ± 30.0 m (range 1.5-185). Aggregation indices calculated from study area indicated that clumping occurred and a simple test of significance for deviation from randomness revealed significant differences for all of eleven sections sampled. Water depth and vegetation density must be considered when evaluating the quality of territory selected by grebes. A study investigating all variables potentially associated with breeding success is recommended. Since aggregations of this size and density are so rare for this grebe species. Whitefish Lake represents a suitable site for future research. There are over 50,000 lakes in Ontario (OMNR) in which only a handful of have been identified to have nesting Red-necked Grebes. Regional and provincial surveys could provide additional data for comparison with Whitefish and other lakes. It is important to establish more specific hypotheses on the habitat and nest site selection of this Grebe and perhaps determine the variables that can be attributed to their breeding success in Ontario.