We investigated the breeding success of a Red-throated Diver Gavia stellata population of approximately 70 pairs during ten seasons, 1991–2000, in a 1820 km2 study area, dominated by coniferous forests and mires in Malung, Dalarna in Central Sweden. The majority of the pairs bred in tarns smaller than 1.0 ha and foraged in larger freshwater lakes or rivers up to a distance of 4.6 km. Average breeding success was 0.76 “large” chicks per pair and year, with a declining trend over the study period accompanied by a decline in the proportion of broods with two “large” chicks. Breeding success was higher in seasons with an average early start of incubation and could be related to higher hatching success and survival of chicks. Predation during incubation was probably an important reason for breeding failures, and abandoning or change of breeding tarns were related to breeding failures. The annual variation of the percentage of broods with two “large” chicks was synchronised between pairs foraging at different fishing lakes, which indicates the influence of some large-scale regional factors linked to the availability of food. Chick survival in two-chicks broods was significantly lower than in one-chick broods. The declining trend in breeding success is an incitement for future monitoring, and any link to impaired foraging conditions needs to be more closely addressed.
During the last 15 years the bird fauna was censused on 2,250 km2 of mires, which is 10–15% of the estimated total area of larger, treeless mires (>0.5 km2) in Sweden. The total number of bird species breeding on mires increases from 25 in the most southern region (Götaland) to 43 in the most northern region (N. Norrland), but more of the northern species breed at very low densities. Among taxonomic groups, species richness of wildfowl and shorebirds increases with latitude while that of passerines shows no clear trend. One possible explanation for the increased species diversity towards the north may be that northern mires generally have a more complex habitat structure, making more niches available. There was no general geographic pattern in the population density of different species. Wildfowl, birds of prey (including owls) and shorebirds (including the Crane) breed at higher densities in the north while passerines have lower densities. One reason for the higher breeding densities of wildfowl and shorebirds in the north is that mires there to a larger extent consist of fens than do mires in the south. Also, a lower nest predation in the north is a probable reason. Combining all species there was no trend in breeding density. Calculations of population sizes show that 25% of the 44 species recorded have breeding populations of more than 10,000 pairs while 25% have less than 1,000 pairs on mires. Estimates of population sizes in Sweden as a whole for all species breeding on mires show that 11 species have more than 50%, and four have more than 75% of their total Swedish population on mires. These species are especially interesting from a conservation point of view since they may be used as indicators of valuable mires.
Most Red-throated Loons Gavia stellata are solitary breeders in small pools and provide fish prey to the chicks from larger waters at a distance seldom exceeding 10 km. More rarely, several pairs nest together in colonies. We compared the breeding performance of solitary and colonial breeders in South-central Sweden during 2000–2016 (except for 2007). Annual productivity was 0.63 and 0.52 chicks per pair and year for solitary and colonial breeding pairs respectively, and mean percentage per year of broods with two chicks did not differ, 24% and 18% (no significant differences). Average hatching probability was the same, 0.576 and 0.581 over years. The average probability however, that at least one chick would be fledged was significantly higher among solitary pairs, 0.915 and 0.731 respectively, although the overall probability of successful breeding did not differ, 0.530 and 0.443. Thus, any benefit with reference to coloniality can be questioned. More likely, colonial breeding was a result of habitat selection, with a concentration of several nesting pairs within a relatively small area with a profitable foraging lake nearby.
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