Text taken from 'Discover Shetland's Birds', Paul Harvey & Rebecca Nason (2015). Published by Shetland Amenity Trust.
The mixing of warm Atlantic water with that of the cooler North Sea is facilitated by the strong tides that flow around Shetland and its windy climate; island residents can expect a gale on 58 days each year! This mixing brings nutrients into the surface levels of the sea and, as daylight lengthens in spring tiny phytoplankton use the sun’s energy to connect these nutrients into food. As phytoplankton numbers increase, so do the zooplankton that feed on them. This forms the basis of a diverse and highly productive marine food web which sustains, among many other organisms, nearly one million breeding seabirds.
Twenty-two species of seabird breed in Shetland, 18 of them in nationally important numbers. Internationally important colonies at Fair Isle, Foula, Hermaness and Noss support the largest numbers and greatest diversity and provide a wildlife spectacle that arguable compares favourably with anything else on Earth. The smaller colony at Sumburgh Head is more accessible and brings you much close to the birds themselves - notably the ever-popular Puffin. An evening trip to Mousa offers the unique experience of visiting a Storm Petrel colony in the walls of a broch - a 2,000-year-old archaeological monument! Indeed, there are few stretches of Shetland’s coastline along which at least a few breeding seabirds cannot be found.
Many of Shetland’s seabirds, including Arctic Skua, Kittiwake, Arctic Tern, Guillemot, Razorbill and Puffin are largely dependent on sandeels (a small, energy-rich fish) during the breeding season. Since the mid-1980s the availability of sandeels in Shetland waters has declined. As a result these species have suffered a series of poor breeding seasons which has subsequently led to population declines. The most recent research suggests that a warming of Shetland waters by as little as one degree Celsius in the winter has driven this reduction in food availability.
Conversely, some of Shetland’s seabirds have managed to maintain, or even increase, they breeding population in the islands. Gannets have the wingspan to forage further afield and are able to take larger fish. Fulmars feed on a wide variety of food items, while Great Skuas have been able to adapt their diet, taking fewer fish and more seabirds, contributing to the decline in the Kittiwake and Arctic Skua populations.
It will be interesting to see how Shetland’s seabird populations fare in the future. At present, however, the declines of most species will be apparent only to regular visitors and residents; from May to July our seabird colonies remain a blur of frenetic activity as adults visit the cliffs to take on the challenges of breeding.
Breeding waders, divers and songbirds
Shetland’s breeding waders and are one of its best kept wildlife secrets. They may attract less attention that seabirds, but a walk across a piece of intact blanket bog or a damp Shetland croft reveals that the density of breeding waders in the islands is now matched by few other places in Britain. Thirteen species of wader breed here and all but two of these are present in nationally important numbers.
The pollen record indicates that scrubby woodland covered much of Shetland until around 5,000 years ago. The climate became cooler and damper at this time, coinciding with the arrival of man. These two factors were responsible for a rapid reduction in woodland cover and the formation of peat, which began to blanket much of the landscape, accruing at a rate of approximately one millimetre each year. Today, blanket bog is a globally important habitat, which stores and sequesters carbon, purifies water and regulates water flow in periods of heavy rainfall. Although somewhat barren in the winter months, it becomes a hive of activity during the short summer. Rich in invertebrates, it provides food for several species of wader, of which the Dunlin is perhaps the one most closely associated with good quality blanket bog. The abundance of lochans (small pools) it supports provides nesting sites for Red-throated Divers.
Much of Shetland’s bog has deteriorated as a result of extensive, historical peat-cutting, accentuated in place by over-grazing. Stock numbers have been reduced in recent years through support from agri-environment schemes, and some areas are showing signs of recovery. The Government is now offering incentives to restore areas of blanket bog.
Due to Shetland’s remote location, geology and climate, agricultural intensification has not proceeded at anything like the pace that it has over much of Britain and, as a result, the islands’ population of some typical farmland species, such as Lapwing, Skylark, Starling and Twite, are still relatively healthy. Traditional crofting, with its mix of pasture and arable, and sheep and cattle, is largely a thing of the past, however. Sheep grazing has become the predominant land use and there are signs that some of our waders, notably Lapwing and Redshank, are now in decline. Several songbird species ceased to breed here in the late 20th century and Twice numbers have declined. Careful targeting of public subsidy will be required in the future if Shetland is to maintain a healthy and diverse population of breeding farmland birds.
Finally, we should mention two very special wader species for which Shetland supports the majority of the British population: Whimbrel and Red-necked Phalarope. The stronghold of the whimbrel used to be the serpentine heaths of Unst and Fetlar, but for reasons which are not altogether clear, it has undergone a rapid decline there. For the moment, however, the population in parts of mainland Shetland is holding up. The RSPB continues to work hard to secure a future for the Red-necked Phalarope in the islands through targeted management work at key breeding sites.
Shetland winters are long, dark and generally wet and windy. Local birdwatchers focus much of their effort on finding rare gulls, venturing further afield to count wildfowl, grebes and divers in sheltered bays, voes and sounds when the wind abates. Slavonian Grebes and Great Northern Divers are easier to locate in calm conditions, and Red-breasted Mergansers feed energetically in the shallows. In late winter, Eiders, Long-tailed Ducks and Goldeneyes start their noisy courtship display. All of these species winter in Shetland in nationally important numbers. Freshwater wildfowl are of less importance in a national context, but good numbers of Wigeon and Teal occur along with a few Whooper Swans. Purple Sandpipers occupy low rocky coastlines and Turnstones often feed on short, damp grassland. Both of these waders are present in Shetland in nationally significant numbers in winter.
In recent years, Shetland has become something of a Mecca for birdwatchers in autumn. This is because the islands have developed an unparalleled reputation for attracting a host of extreme vagrants from Siberia and North America. Such birds are often thousands of miles away from their intended winter destination - Southeast Asia, in the case of many of those from Siberia, and Central or South America for those originating from North America. The precise mechanism for this vagrancy remains unconfirmed. Some North American migrants fly out over the Atlantic on their way southwards to the Caribbean or Central America. If these encounter depressions (cyclones) tracking rapidly eastwards, they can get caught up in strong westerly winds, bringing them across the Atlantic to Europe.
A different mechanism might be operating for those from Siberia. These are often termed reverse migrants, the theory being that young birds possess a faulty internal compass and, rather than migrating towards their wintering grounds, they migrate in the opposite direction; Shetland is these well placed to receive them. Look at a globe, rather than a flat map, and this will make more sense.
Of more relevance to this book, however, are the common and scarce migrant that turn up in Shetland in reasonable numbers every year. These species make regular seasonal movements; in spring they head north to take advantage of the rich bounty of food available during the northern summer, but in autumn they head back south to avoid the hardships of a northern winter. These migrants can be divided broadly into two groups. The first of these comprises species that breed to the north-west of Shetland, in Faroe or Iceland, or perhaps Greenland. This includes Whooper Swan, Greenland and Pink-footed Goose, several duck and wader species, Redwing, Wheatear, White Wagtail, Meadow Pipit and Snow Bunting. Many of these intend to use the islands as a migratory stop-over at which they can refuel before heading further south to their winter quarters. Some of the larger species, notably geese, may just use the islands as a navigational tool to guide them on their way south.
The second group involves the vast majority of species, and these arrive in Shetland largely by accident. Most of these are birds moving between their summer breeding grounds in Scandinavia and their winter quarters in south-west Europe, the Mediterranean or Africa. As they head across the North Sea or Baltic, they may run into strong winds from an eastern quarter, sometimes accompanied by poor visibility and/or rain. As a result, they drift across the North Sea, or get completely disoriented, and may end up in Shetland. Large arrivals (falls) occur when significant numbers of birds get caught out on their way. If Shetland were not here, it is quite possible that many of these birds would eventually be lost at sea. The number of migrants reaching Shetland is much greater in autumn than in spring. This is for two reasons: there is a far bigger pool of birds in autumn after the breeding season, and many of the inexperienced juveniles will commence migration even in adverse weather. Come spring, the pool of birds will be much smaller, following winter mortality of many juveniles, and those birds that do remain are far more experienced at migrating. It should come as no surprise, then, that the majority of migrants that reach Shetland in autumn are young birds.
In addition, there are two other mechanisms that are likely to result in migrants reaching Shetland: the first, which we have already dealt with, is reverse migration, which may explain why some species that breed in central Europe (e.g. Barred Warbler) or western Asia (e.g. Yellow-browed Warbler) turn up in Shetland in surprisingly large numbers every autumn; the other is spring overshooting. This theory, for which there is considerable circumstantial evidence, argues that in fine, anticyclonic spring weather, some migrants travel further north, or north-west, than they had intended to, thus overshooting their normal breeding grounds. Such overshooting could also be deliberate, however; migrants that are unable to secure a breeding territory within their normal range may head further north in order to try and obtain one. These individuals are most likely to be young males.
One other group of birds that we have yet to consider is irruptive species. These are not migrants in the normal sense of the word, as their movements are not regular and often do not involve a return leg. Most irruptive species that occur in Shetland originate from the forests of Scandinavia and further east. They usually occur here when a shortage of food in the breeding area coincides with high population levels. Birds, mostly less-experienced juveniles, disperse to the south and west to look for food. Great Spotted Woodpeckers, Waxwings and several finches and tits are typical irruptive species. By whatever means they arrive, the excitement of looking for migrants in bushes or on a coastal headland is a real passion for many birdwatchers, and a large fall of migrants, albeit a rare occurrence, will remain in the memory for a long time.