Abstract
Lake Vombsjön in southern Sweden is visited by large numbers of Great Crested Grebes Podiceps cristatus (>2000), Great Cormorants Phalacrocorax carbo (sometimes >1000) and Common Mergansers Mergus merganser (up to 2000) in late autumn and early winter. Different species exploit them. Great Crested Grebes are used especially by commensal Common Gulls Larus canus; the gulls take advantage of fish that flee towards the surface. Common Gulls also use cormorants and mergansers in the same way but also try to kleptoparasitize them. Both Herring Gulls Larus argentatus and Great Black-backed Gulls Larus marinus kleptoparasitize these two species, while Red Kites Milvus milvus, Grey Herons Ardea cinerea and Carrion Crows Corvus corone use them commensally. White-tailed Eagles Haliaeetus albicilla seem to use both methods to obtain fish. On 50% of one hundred visits during November to March, eagles were seen flying low over the fishing flocks. They would fly a metre or so above the flocks and then accelerate and attack a bird holding a fish. The bird would then either try to escape by a rush or by diving, dropping the fish which the eagle seized. Interestingly, the flock-fishing birds showed no fear reactions towards the eagles but appeared to regard them similarly to large gulls.
Highlights
All three species are exploited by several other bird species that try to obtain a share of the fish these species capture or make accessible
Lake Vombsjön has a maximum depth of c.15 m, 13% are more than 10 m deep, and 55% are between 5 and 10 m deep while 30% are less than 5 m deep
Great Crested Grebes were used predominantly by Common Gulls Larus canus that circled above fishing grebes, in particular over denser aggregations, and plunge-dived among the grebes
Summary
Lake Vombsjön has a maximum depth of c.15 m, 13% are more than 10 m deep, and 55% are between 5 and 10 m deep while 30% are less than 5 m deep. The lake has a rich fish fauna with perch Perca fluviatilis, roach Rutilus rutilus, bream Abramis brama, björkna Blicca blicca, rudd Scardinius erythrophthalmus, ruffe Gymnocephalus cernua, bleak Alburnus lucidus and others. The shores are rather low and covered with deciduous trees except in the south where the shores are steeper and border to a large pine plantation, Vombs Fure. Suitable observation points exist both in the west (including a bird observation tower) and in the north-east (Öveds badstrand) and east. There are embankments along the shore from south-east to north-east, and in north and west
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