Parental feeding in the Red‐throated Diver Gavia stellata

Abstract

We describe parental feeding by Red‐throated Divers Gavia stellata from 14.2–44.7 h of observation on each of eight pairs, coordinated with simultaneous observations at potential fishing lakes, in southwest Sweden. All pairs bred on small tarns (0.8–10 ha), normally devoid offish, and foraged in larger freshwater lakes 1.9–7.7 km away from the breeding site. Most pairs directed the majority of the foraging trips to the same lake. On average, parents made seven foraging trips per day per young. Both parents contributed to feeding activities and, in broods of two young, each chick obtained an equal amount of food. Cyprinid fish (mainly Rutilus rutilus) and salmonids (mainly Coregonus albula) dominated. Qualitative predictions from central‐place foraging theory concerning relationships between the time for a foraging trip and the size of prey delivered by a single‐prey loader were not corroborated. The amounts of fish biomass delivered were in agreement with or lower than predictions from models on daily energy requirements; this result is related to the decreased abundance of fish in acidified freshwater lakes and the decline of Red‐throated Diver populations in south Sweden.

Spatial and temporal variation in foraging of breeding red‐throated divers

“…Finland Iceland freshwater (Eriksson et al 1990, Eriksson and Sundberg 1991, Duckworth et al 2020b) and marine environments during the breeding season (Reimchen and Douglas 1984, Black et al 2015, Rizzolo et al 2015. As predicted, all diving metrics (dive depth and dive duration) differed between Finland and at least one of the other two sites (Table 1).…”

“…Instead it appears that the tracked divers may have adapted their foraging strategy via prey selection, rather than increasing the number of foraging trips or foraging intensity through time. This is suggested by earlier observations of RTDs from Sweden, where it was found that the number of foraging trips did not change as the breeding season progressed, but instead the size of the prey brought back by the parents changed (Eriksson et al 1990). This effect was particularly noticeable when the chicks were very young (Reimchen and Douglas 1984).…”

“…RTDs in Finland breed at a much greater distance from the coast than the majority of those breeding in Scotland and Iceland, and previous evidence from populations breeding in similar environments further from the coast show RTDs to be likely to forage in freshwater habitats (Eriksson et al 1990, Eriksson and Sundberg 1991, Duckworth et al 2020b. In contrast, birds breeding close to the coast, tend to forage in marine environments (Reimchen and Douglas 1984, Black et al 2015, Rizzolo et al 2015.…”

“…This aversion to anthropogenic presence could be detrimental to demographic rates, through displacement effects (Drewitt and Langston 2006), but these effects are hard to observe and measure directly, as the birds are often in inaccessible locations. Some information exists on aspects of the behaviour and ecology of this species at the nest during the breeding season, such as descriptions of chick rearing and nesting success (Eriksson et al 1990, Rizzolo et al 2015. However, there is a lack of information on foraging behaviour and water depth usage compared to other, better studied species (Grémillet et al 1998, Linnebjerg et al 2014, Amelineau et al 2019, Poupart et al 2019.…”

Smålommens Gavia stellata häckningsframgång i relation till vattenkemi och fiskbeståndens sammansättning i olika fiskevatten

“…In contrast, the Red-throated Diver Gavia stellata breeds at small lakes and tarns (mostly <1ha) often devoid of fish, and the non-fledged chicks are fed with fish caught by the parents in neighbouring larger lakes or at sea up to a distance of almost 10 km. Furthermore, the Red-throated Divers are more restrictive in their choice of fishing lakes than the Black-throated Divers; the chicks are primarily fed with cyprinid and salmonid fish, and abundance of prey fish is as important as distance to alternative foraging lakes (Eriksson et al 1990, Eriksson & Sundberg 1991, Eriksson & Johansson 1997. For pairs breeding in coastal areas, e.g.…”