Seasonal changes in Common Scoter distribution

Back in the 1970s, sea-watching avidly from Ynyslas in mid-Wales into Bae Ceredigion, often through watery eyes from looking into fierce cold winds, I desperately tried to understand why the rafts of Common Scoter Melanitta nigra sitting among the swirling waves distributed themselves where they did and how and why their positions changed throughout the winter. Featureless waves and parallax gave no impression of the birds’ true spatial arrangement, so long before the invention of drones, I dreamt of being able to get above them to gain an impression of how they distributed themselves and where and upon what, they fed. Some years later, Mick Green and Chyfeillion Bae Ceredigion (the Friends of Cardigan Bay) very kindly chartered a high winged aircraft and indulged me in the unforgettable experience of flying transects across the northern two thirds of Bae Ceredigion, from Aberystwyth north to Pen Llŷn. Suddenly, viewed from above, I realised in my nativity the sheer numbers of these black ducks that we simply could not see from land, and yet how densely they could aggregate. What intrigued me then, as now, was that if Common Scoter could gather in such numbers over shallow marine waters to feast literally for months on end upon the poor helpless immobile bivalves lying in the sandy substrates below, then surely, they must severely deplete the density of their food items during winter? This might also provide a potential explanation for changes in distribution as the season progressed.

Figure 1. Part of a moulting flock of Common Scoter photographed in Aalborg Bugt © Rasmus Due Nielsen.

Fast forward to the late 1990s and Denmark was in the throes of the first environmental impact assessments of developing offshore turbine windfarms and our institute was charged with mapping the distribution of: guess what? Among many other species, Common Scoter. This species was being counted from systematically flown aerial transects in wintering numbers regularly exceeding 100,000 in Aalborg Bugt, a huge shallow sheltered brackish water bay with sandy substrates off the east coast of Jutland, where I live. I could hardly believe my luck! Even more lucky was the fact that, because the species was present throughout much of the year, we had the good fortune to secure funding to assess their numbers and distribution through the key parts of the annual cycle. Now we could see if the birds changed their distribution through the course of the scoter year: and sure enough, they did. Optimal foraging theory would predict that, on arrival, they would feed over the densest concentrations of their food resource in the shallowest waters to maximise dietary intake and minimise the diving costs to access their food. We predicted that they would initially feed on their largely immobile bivalve prey in shallowest water when the first males started to aggregate to moult. Later, as more moulting birds arrived, to be joined later initially by failed breeding females and later still by breeding females and first year birds, we expected that the flocks would spread out into deeper and deeper water and disperse over a larger area as winter progressed. And so it turned out: first birds densely aggregated in shallow waters, but as numbers built up, so the Common Scoters dispersed into deeper waters, at lower densities, more and more widespread but within the same area.

So far so good, but no researcher likes to conclude very much when n = 1 for number of study sites. Fortunately, ten years later, during our collaboration with Johanna Osterberg’s (née Kottsieper) PhD project based at Stefan Garthe’s group at Kiel University in Germany, we were able to scrutinise a similar system in the German Wadden Sea, where Common Scoter also aggregate in their tens of thousands to moult and winter (Kottsieper et al. 2019). More exciting still was the fact that this North Sea environment was subject to more strongly lunar tides and offered a more exposed marine system with a muddier substrate than the more sheltered, sandy and brackish Baltic setting of Aalborg Bugt. Potentially also with a completely different food supply (Kottsieper et al. 2019). But would the Common Scoters show the same patterns? Sure enough, as we finally now report in Fox et al. (2025), the ducks there also started to aggregate in the shallowest waters, distributing themselves over larger areas and deeper water as numbers built to mid-winter, ending up on average in deepest waters and most widely dispersed in the spring.

Alas, sequential sampling of the benthos infauna of the substrates in both areas has turned out to be both time consuming and expensive, so in neither area has it been possible to follow what is happening to prey size class frequencies and their densities as the season progresses and scoter numbers increase. As a result, our original hypothesis relating to prey depletion has unfortunately yet to be rigorously tested, but that remains the next research priority for the future. Likewise, we cannot know whether the birds shift to deeper waters to take more profitable but different prey later in the season, as is the case with moulting Surf Scoters M. perspicillata in Canada that started feeding in shallow water on clams and cockles in soft substrates but shifted to urchins and blue mussels on harder substrates offshore later in the season (Gilliland & Savard 2021). Similarly, we do not know how much intra-specific agonistic interactions may influence flock size (which diminishes with season) and dispersal (which increases) and which may also play a part in shaping these patterns. So, we still need to establish the cause behind these distinctive patterns. Nevertheless, it has been a very pleasing and unique experience to be able to document such changes in the distribution of literally hundreds of thousands of ducks over two contrasting areas amounting to thousands of square kilometres of open sea, well out of sight of land and to bear witness to their consistent patterns in relation to water depth and distribution in different years and in very different marine environments.

References

Gilliland S.G. & Savard, J.P. 2021. Variability in remigial moult chronology and nutrient dynamics of Surf Scoters, Melanitta perspicillata. Wildfowl 71: 193–220.

Kottsieper, J., Schwemmer, P., Markones, N., Fox, A.D. & Garthe, S. 2019. An invasive alien bivalve apparently provides a novel food source for moulting and wintering Common Scoter Melanitta nigra the German Bight. Heligoland Mar. Res. 73:11.

Schwemmer, P., Volmer, H., Enners, L., Reimers, H.-C., Binder, K., Horn, S., Adler, S., Fox, A.D. & Garthe, S. 2019. Modelling distribution patterns of common scoter (Melanitta nigra) by its dominant prey, the American razor clam (Ensis leei) and hydrodynamic parameters. Est. Coast. Shelf Sci. 225: 106260.

Image credit

Top right: Pair of Common Scoter © Daniel Bergmann.