The ecology of breeding Common Scoters

Project objectives
Work planned or underway
Results
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Flock of common scoters sleeping

Common Scoters are diving ducks, perhaps best known in Britain as winter migrants from northern breeding grounds. They commonly form large flocks of "sea-ducks", which characterise sandy bays and firths around the British coastline in winter. They are much less well known as breeding birds, typically being very secretive at their often remote upland nesting lakes.

At one time, at least 200 pairs nested in the British Isles, from Shetland to western Ireland. However, steep declines were recorded in Northern Ireland from the 1970s, and, more recently, two national surveys showed that the British population roughly halved between 1995 and 2007, from 95 to 52 breeding females. These declines, and the rare, localised nature of breeding scoters, have resulted in the species being red-listed. Although the common scoter is a northern breeding species, our breeding birds form part of an important population. The Flow Country population is 600 km north of the southern edge of the breeding range. And in the mid 1990s, breeding densities (pairs per 50 km2) in the British Isles were within the range of values estimated for Finland.

Project objectives

To measure the differences between preferred scoter lochs and other, similar lochs, in terms of food supply, food accessibility and predator abundance.
Using detailed foraging watches, to determine key foraging habitats within lochs, and periods of high foraging effort within the season.
To use these results to propose conservation measures.

Work planned or underway

Fieldwork took place over three seasons, April-August 2009-11. Two comparable sets of 13 lochs, one regularly used by scoters, and one now rarely used, were compared in two study areas: the Flow Country and the West Highlands. At each loch, we measured the abundance of aquatic invertebrates, predators, fish, and physical factors like substrates and depths. Scoter foraging behaviour was quantified during standard watches of adults and broods.

The laboratory work for the project (sorting, counting, and measuring samples of aquatic invertebrates) took place during winter 2011/12, and the project is now in the writing up phase.

Results

Fieldwork has now been completed, and some preliminary results are given here. These will be analysed in more detail prior to seeking publication in the peer-reviewed scientific literature in due course.

A total of 257 one-hour foraging watches were completed. These showed that females foraged most intensively on arrival in breeding areas, when they spent 41% of their time actively foraging. Despite their egg laying requirements, female foraging effort was, unexpectedly, no higher than that of males. Females with broods spent less time foraging, for example only 22% in the late brood period, and much time guarding ducklings. Duckling foraging effort was usually higher than that of adults, and rose from around 41% of time for young ducklings, to 53% for older ducklings. These results emphasise the importance of early- and late-season food availability, and may imply that females rely on pre-breeding marine areas to gain body condition before breeding.

Near-shore areas were strongly preferred for foraging. Foraging females and ducklings were found, respectively, about three and five times as often within 10 m of shore as expected by chance; these areas typically being under 1 m deep. Diving and dive-recovery took up the overwhelming majority of foraging time (e.g. 67% and 28% respectively for adult females). Only newly-hatched ducklings regularly used other behaviours, spending 10% of their foraging time 'head-under' or 'surface-gleaning'.

In the Flow Country, there was often higher invertebrate biomass and larger invertebrates at preferred scoter lochs compared to other lochs. In the West Highlands however, invertebrate abundance was much lower at standard sampling points than in the Flow Country, and showed a weaker pattern in relation to scoter use. At preferred scoter lochs, compared to other lochs, trout were less abundant, larger, grew faster, and had more invertebrate food in their stomachs at a given size. This pattern would be consistent with invertebrate food availability for both scoters and trout being lower where trout were more abundant.

Surprisingly, mammalian predator frequency was about twice as great at preferred scoter lochs, than at other lochs. Despite this, there was some evidence of higher breeding success (within the group of preferred scoter lochs) where mammalian predators were less abundant (see below). Avian predator frequency on the other hand showed little difference between the two groups of lochs. Pine marten and fox were the most regularly recorded mammalian predators, and common gull and black-throated diver (the latter included due to its sometimes severe aggression towards ducks) the most regular avian predators. American mink was the third most regularly recorded mammalian predator in the West Highlands, but is at present absent from the Flow Country.

Physical variables showed some differences between preferred scoter lochs and other lochs. In accordance with expectations (given scoter foraging behaviour) preferred lochs had near-shore zones with water about 20% shallower, and about 44% more soft substrates. However, contrary to expectation, water temperatures on preferred scoter lochs were not generally any lower than those of other lochs, suggesting for example that emergence times for aquatic invertebrates might be similar on the two groups of lochs.

Preliminary statistical analyses suggested that numbers and occurrence of adult female scoters were linked strongly to high invertebrate abundance in shallow water. Some importance for large invertebrates and few avian predators was also supported by analyses. Numbers of ducklings per female tended to be higher where there was abundant and large-bodied invertebrate food, soft substrates, and few mammalian predators. These analyses will be further tested and developed before submitting the work to peer-review and scientific publication. Potential routes to improving scoter habitat quality will be discussed and developed, with options including trout or water level management, and mink control.