Organochlorines, Predators and Reproductive Success of the Red-Necked Grebe in Southern Manitoba

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Abstract
I assessed factors influencing the reproductive success of Red-necked Grebes (Podiceps grisegena) in Turtle Mountain Provincial Park, Manitoba, during 1980 and 1981. Nesting losses were high as 78.9% of 697 eggs in 179 regularly observed nests failed to hatch. An additional 4.4% disappeared during the hatching process. Similar success rates among 137 eggs in 29 infrequently observed clutches indicated that observer presence did not adversely affect nesting success. Losses due to waves (2%) and intraspecific territoriality (6%) were minimal. Predators, particularly raccoons (Procyon lotor), accounted for half of the losses. Organochlorine residues contributed to many of the remaining losses. Average DDE levels (74.3 ppm, lipid weight) in 12 eggs were comparable to those reported in other recent studies, but average PCB levels (194.8 ppm) were unusually high. Average residue levels of 1 to 8 ppm were recorded for mirex, dieldrin, hexachlorobenzene, oxychlordane, heptachlor epoxide, and DDD. The average Ratcliffe Index (1.72) for 29 eggs collected during this study was significantly less than Ratcliffe Indices for 112 Red-necked Grebe eggs collected in North America prior to 1947 (x̄ = 1.84) or for 186 eggs collected in central Alberta in the early 1970s (x̄ = 1.86). Other indications of thin-shelled eggs included the frequent disappearance of single eggs between nest checks (9%) and cracked eggs noted in several active nests. The estimated rate of egg inviability (25%) exceeded the 5% rate considered normal for most grebes. Egg inviability contributed to at least 23% of 191 eggs in 58 successful clutches being deserted after partial clutch hatching. A high incidence of renesting, 83.5% of 79 failed pairs, resulted in 51% of 114 breeding pairs hatching at least one young. An average of 2.5 young hatched per successful pair. This was reduced to 1.9 young per successful pair or 0.9 young per breeding pair at fledging. Fledging success was lower than recorded in most other Red-necked Grebe studies. Organochlorines, recent increases in unnatural predators, and human recreational activities have probably contributed to reduced productivity and population declines among Red-necked Grebe populations throughout North America.

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