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We compared the breeding success of Black-throated Divers Gavia arctica inside and outside bird sanctuaries with restricted public access during the incubation period in Lake Fegen-Svansjöarna and Lake Sottern, South Sweden. Both lakes held populations of approximately 15–20 pairs. At Fegen-Svansjöarna, average breeding success was 0.51 large chicks per pair and year inside and 0.27 outside the sanctuaries (field surveys in 1983—1984 and 1997—2000). The difference was not statistically significant, however, and in two out of the six study years breeding success was higher outside the bird sanctuaries. At Sottern, breeding success was 0.39 and 0.25 large chicks per pair and year inside and outside the bird sanctuary, but breeding success was higher inside the sanctuary only in four out of nine years, 1997—2005. The results from the two lakes together indicated an overall positive effect (P = 0.08). We recommend keeping a constant water-level during incubation to be the primary measure to enhance breeding success. Sanctuaries with limited access may be an additional benefit at lakes highly frequented for out-door recreation.

We investigated concentration of mercury in eggs of Red-throated Diver Gavia stellata and Black-throated Diver Gavia arctica collected in South and Central Sweden, 1981—1997. In South Sweden (Sydsvenska Höglandet), mean concentration in Red-throated Diver eggs was remarkably high (6.84 ppm dry weight, 5 pairs), or more than three times as high as in Black-throated Diver eggs (2.11 ppm dry weight, 16 pairs). In Central Sweden (Svealand), the concentration in Red-throated Diver eggs (3.04 ppm dry weight, 8 pairs) was significantly lower than in South Sweden, but still higher than in Black-throated Diver eggs (1.21 ppm dry weight, 5 pairs). In Black-throated, but not in Red-throated Diver, there was higher mercury concentrations in eggs from pairs fishing in lakes affected by acidification. We found no relationship between mercury and breeding success, although concentrations in single Red-throated Diver eggs exceeded the level associated with high risk of impaired reproduction. We recommend that mercury concentration of diver eggs should be further monitored, with reference to the risks of increased exposure, related to leakage from terrestrial deposits of air-borne transmissions to freshwater habitats.

During the latest decades, Canada Geese have increased in numbers and expanded over large parts of north-western Europe. Hitherto, the risk for adverse effects on the native bird fauna has been assessed to be negligible, although there are single records of divers (Gavia spp.) having failed or abandoned the breeding site after the establishment of Canada Geese. Therefore, we initiated an inquiry about breeding success and nest locations of divers and geese in Sweden. For Red-throated Diver, we compared the breeding success for pairs nesting at tarns with and without breeding Canada Geese. We found no indication of any negative effects on breeding success: The average production was 0.82 “large” chicks per pair and year for pairs at tarns with breeding Canada Geese, compared to 0.75 at tarns without breeding geese. For Black-throated Diver, the average hatching success for pairs with and without nesting Canada Geese on the same islet was 46% and 47%, respectively. But the average distance to the closest goose nest (on another islet) was lower for failed nests compared to successful ones. We judge, that the expansion of the Canada Goose is not a threat on a national or regional scale, although single diver pairs certainly suffer from having Canada Geese as close neighbours. Among both species of divers there are also indications of adaptations to the geese, e.g. through moving to new nesting sites at the same breeding lake. Finally, potential benefits from the presence of Canada Geese, e.g. if the geese keep other potential predators away, should not be overlooked.