8 female Common Scoter,
Melanitta nigra, were initially pointed out to me flying and then mixing with Wigeon. You can spot them quite easily from a distance by picking out their pale faces, which Velvet and Surf just don't have. I rather thought that they had slightly paler bellies as well, which might, or might not, indicate that they might have been first winter birds.
I hadn't realised that a few Common Scoter actually breed in Scotland and Ireland, on inland lakes as indeed they do elsewhere. In Scotland it tends to be quite variably sized lochs in the North and West, particularly in the Flow Country in Caithness and Sutherland. It is possible that they didn't breed in Scotland much before the mid 1800s. In 1995 there were about 100 pairs in Ireland and 100 pairs in Scotland, down slightly from the 70s. However, the main breeding range of the nominate race,
nigra, is across the Arctic Ocean coastlands of Northern Europe and Russia, with the other,
americana, race further to the east, and across towards Alaska. It is quite possible of course that subspecies
americana is actually a separate species, recognisable by differences in call/song - some US research is treating it as such, and the division has been accepted by the BOU since the mid-noughties.
Threats might include eutrophication, competition for food, or predation by mink (encouraged by conifer planting in Scotland perhaps). A study in Scotland, triggered by a post-1995 more than halving of the population, indicated that lakes with shallow water around the edges, containing larger invertebrates, with more Three-Spined Sticklebacks but fewer Brown Trout, held significantly more birds. It was suggested that the water levels in hydro-electric lakes could be lowered, and also that perhaps more fishing in trout lakes could be encouraged! Many small trout might be worse than a few large ones.
Age of first breeding is 2 - 3 years. There is a monogamous pair-bond for the season, which may start in the winter flocks, with increasing pairing on the breeding grounds. The nests are usually built by the female, concealed close to the water's edge. It is hollow, lined with grass, moss, lichen and down. The male tends to defend the female wherever she may be. Egg-laying date is determined by timing of the arctic thaw, 6 - 8 eggs laid at 1-2 day intervals. Incubation is for almost exactly a month, with the eggs covered by down if the female leaves the nest. Hatching is quite synchronous. However, the male often deserts the female soon after the start of incubation, and the female then tends the young until they fledge - broods may sometimes be amalgamated. The young are nidifugous and precocial. They are generally self-feeding, but cared for and brooded at night by the female while still young. They fledge after roughly 45 - 50 days.
Common Scoters tend to over-winter in large flocks, hundreds or even thousands. Around the UK the winter population is about 100,000 with main concentrations in Carmarthen and Cardigan Bays, the Moray Firth and the North Norfolk coast. A previously unknown concentration on Shell Flat in the Irish Sea off the NorthWest coast was discovered by aerial survey in preparation for proposed windfarms. Most of the over-wintering birds generally stay between October and March. Males tend to concentrate more in the north, females and immatures apparently have to fly further south. After breeding, the birds move to moulting areas, for example Northern European and Russian birds moving into the Baltic. Then they move on to the over-wintering areas, for example the North Sea and the coasts of Britain. Numbers in the Waddensee appear to have dropped from about 40,000 in the 1960s, to about 1,000 now - but could this be due to migration shortening? The East Siberian/American race by contrast over-winters along the coasts of North America. The return journey was also studied in Surf Scoters on the west coast of North America. Data from birds tracked over 2 years indicated strong migration route fidelity, but altered chronology and stopover locations between years. Departure date varied by wintering site, but arrival and apparent settling dates were synchronous, suggesting individuals adjusted migration timing to meet an optimized reproductive schedule. Canadian research showed a positive correlation between longer snow cover and population size in Scoter species, indicating that climatic warming might reduce survival - but how were populations measured?
The Common Scoter feeds on shellfish obtained by diving, generally in daylight, from smaller groups, occasionally group-diving. Other birds or even fish such as introduced Perch on inland waters may compete with the Scoters for food. They may loaf from time to time during the day, and roost at night. A wide range of molluscs are taken such as mussels, cockles, clams, whelks, etc. Also some crustacea and tube-worms. There was strong evidence (Bangor University) that the maximum observed biomass of bivalves occurred at a mean depth of c. 14 m off the Lancashire coast and at c. 8 m off the north Wales coast. This coincided well with the distribution of Common Scoter at Shell Flat, but less well with the distribution of birds off North Wales. In Holland, research indicated that a decline in the bivalve
Spisula was perhaps linked to the long-term decline in Common Scoter.
There is a lot of data on the Common Scoters in the Solway Firth - in
British Birds here.
Foraging behaviour can be varied. One Canadian paper stated that the distribution of predators is widely recognized to be intimately linked to the distribution of their prey. Foraging theory suggests that predators will modify their behaviors, including movements, to optimize net energy intake when faced with variation in prey attributes or abundance. While many studies have documented changes in movement patterns of animals in response to temporal changes in food, very few have contrasted movements of a single predator species naturally occurring in dramatically different prey landscapes. We documented variation in the winter movements, foraging range size, site fidelity, and distribution patterns of a molluscivorous sea duck, the Surf Scoter (
Melanitta perspicillata), in two areas of coastal British Columbia with very different shellfish prey features. Baynes Sound has extensive tidal flats with abundant clams, which are high-quality and temporally stable prey for Scoters. Malaspina Inlet is a rocky fjord-like inlet where Scoters consume mussels that are superabundant and easily accessible in some patches but are heavily depleted over the course of winter. We used radio telemetry to track surf scoter movements in both areas and found that in the clam habitats of Baynes Sound, Surf Scoters exhibited limited movement, small winter ranges, strong foraging site fidelity, and very consistent distribution patterns. By contrast, in mussel habitats in the Malaspina Inlet, Surf Scoters displayed more movement, larger ranges, little fidelity to specific foraging sites, and more variable distribution patterns. We conclude that features associated with the different prey types, particularly the higher depletion rates of mussels, strongly influenced seasonal space use patterns. These findings are consistent with foraging theory and confirm that predator behavior, specifically movements, is environmentally mediated.
Heavy shipping appeared to negatively impact on Scoter distribution in Canadian research. Off-shore wind-farms could perhaps impact either positively or negatively. On the other hand tanker spills definitely have impact: After the Sea Empress oil spill in Carmarthen Bay, numbers crashed for the next three years, but then recovered to the level of the previous population. The NAO appeared to impact upon distribution of Scoters of the east coast of North America. On the other hand, off-bottom oyster culture structures offered opportunities for mussels to settle in large numbers, and thereby increase feeding opportunities for Scoters, and its benefits have been demonstrated to increase habitat choice off British Columbia.
In a study of Surf Scoter diving activity in Canada, variation in both hourly and daily foraging efforts was best explained by date only, as opposed to substrate nature. Effort per hour was lowest in early December (presumably owing to very high prey abundance), increased until mid-February as prey declined, and then decreased again in March (probably owing to increased daylight time for foraging). Foraging effort estimated over a full day increased steadily from December to March as prey were depleted. Temporal patterns of effort did not vary by habitat (commercial mussel beds with easy to access shellfish, rapidly depleted upon harvest) after accounting for seasonal effects. Instead of increasing foraging effort in habitats with strong depletion, Surf Scoters redistributed to habitats with lower degrees of prey reduction as the season progressed. We suggest that Surf Scoters respond to variation in prey by adjusting both foraging effort and habitat selection as the prey landscape changes.
The duckling diet is quite varied, including insects and seeds. Looking at correlates of distribution of other species of Boreal waterbirds such as White-winged Scoter, Scaup and a Grebe, the presence of amphipods in larger lakes (>25 Ha) appeared to be the main interaction promoting occupancy and nest building.
It is thought that gluco-corticoids may influence weight gain, activity and breeding status. But what controls the gluco-corticoids?
Aggressive behaviour: Males may dash at each other (skating) with neck outstretched. Actual violent combat rare.
There is also a head-shake.
Several groups of Dark-bellied Brent Geese, including about 65 in one larger group gathered just off-shore in the gloom as I walked back.
