| Habitat and Ecology: |
Behaviour Most of this species is fully migratory although it may only travel short distances2, and certain populations in the north-west of Europe may also be sedentary1. The species breeds from April in solitary pairs1, after which it undertakes short northerly moult migrations to coastal areas, large lakes and rivers2 to undergo a period of flightless moult lasting 3-4 weeks (males leave for this moult migration first while females are still incubating)5. Large moult gatherings are common during this period, males arriving at such gatherings in early-June and numbers peaking in late-August when adult females arrive5. The southward autumn migration begins in late-August with most arriving in the winter quarters by early-December5. Females tend to migrate further than males3, 5 and juveniles migrate further than adults5. The return migration to the breeding areas occurs as early as mid-February5, the species timing its arrival to coincide with the thawing and appearance of open water2. Non-breeders may also oversummer on wintering grounds3. The species is gregarious outside of the breeding season2, 6 usually being observed in small scattered groups5 or in small flocks on migration2. Several hundred individuals may roost together6 and large flocks often gather to feed at sewage outfalls1 during the winter, although the species rarely occurs in very large flocks5. Habitat The species is restricted to water close to the shore and less than 10 m deep5 (showing a preference for waters 4 m deep)6. Breeding When breeding the species shows a preference for oligotrophic lakes devoid of fish2 but with abundant invertebrate life4, and requires tree-holes (or artificial nestboxes) for nesting1. Suitable habitats include freshwater lakes, pools, rivers1 and deep marshes4 surrounded by coniferous forest1. Non-breeding The species winters mainly at sea5 on inshore waters1, shallow bays2, estuaries1 and coastal lagoons1, 5, especially in the vicinity of sewage outfalls1. Further to the south and on migration the species may also frequent large rivers, lakes2, 5 and reservoirs5. Diet The diet of the species consists predominantly of aquatic invertebrates such as molluscs, worms, crustaceans, aquatic insects and insect larvae1 (e.g. dragonflies, damsel flies and may flies)4, as well as amphibians, small fish1 and some plant material (mainly in the autumn) such as seeds, roots and the vegetative parts of aquatic plants1. Breeding site The species nests in hollows of mature trees1 (e.g. aspen, spruce or oak)7 formed by woodpeckers or by bacterial or fungal heart-rot invasions2 that have internal cavity diameters of c.20 cm (although the height of the hollow does not seem to be important)4. The species will preferentially nest in trees in open stands near water3, 4 or solitary trees on the edges of marshes4, rather than in trees in dense stands (to increase the ease of entry by flying)4. The species will also nest in artificial nestboxes1. Management information In Scotland, UK the introduction of a sewage treatment scheme in the Firth of Forth (a large marine bay) resulted in a considerable reduction in the abundance of the species, with feeding flocks only remaining at outfalls where sewage continued to be discharged in large quantities9. It was unclear whether the changes in the species's distribution were due to reductions in the number of food items borne in the sewage or to reductions in aquatic invertebrate abundance as a result of the new treatment system9. In some areas nestbox erection programmes have been shown to cause significant range expansions and population increases1, 11, although an experiment in southern Finland found that even though nestbox provision increased breeding numbers of the species there was a negative density-dependent effect on reproductive output (i.e. the number of fledged young did not increase despite an increase in breeding pairs)10. Nesting habitats in general may also benefit from a more extended rotation of timber harvesting2.
|
