The Status of Seabirds in Britain and Ireland

Clare Lloyd, Mark L. Tasker, Ken Partridge

• Publisher: T & AD Poyser
• Year: 2010
• Print ISBN: 9781408138007
• eBook ISBN: 9781408137994
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Zoology
• Index: Biological Sciences

The Status of Seabirds in Britain and Ireland book presents the most up-to-date information available on these seabird populations, their numbers and distribution, and their changing fortunes in recent years.

The oceanographic and coastal features of the British Isles provide a wealth of ideal habitats for breeding seabirds and it is no surprise that the British and Irish seabird populations are of international importance, both in sheer numbers and in species diversity. Indeed, for some species British waters are host to the greater part of the world's population.

The Operation Seafarer survey, carried out in 1969-70, provided a baseline for future work that led to the establishment of the Seabird Colony Register by the Seabird Group and the Nature Conservancy Council. The results and analysis of their counts during 1985-87 form the basis for this book. Improved census methodology and a new computerised database has set the standards for seabird monitoring in future decades.

In Part 1 the general biology and population trends of British seabirds are described to set the scene for Part 2, in which the results and analysis for each of the 24 breeding species are given in detail. Full descriptions of the counting methods and the estimating factors used provide guidelines for future surveys not only in Britain, but wherever seabirds are of interest and importance.

No-one with an interest in seabirds or conservation can afford to be without this authoritative book, nor but be grateful to the small army of professionals and amateurs who have so ably explored our coastal habitats.

Illustrated by Keith Brockie.

PART 1

Seabird Biology and the

Seabird Colony Register

Seabirds and their Breeding Populations

Every summer many millions of seabirds use the coasts of Britain and Ireland for breeding. Some arrive after long migratory journeys from as far away as Newfoundland, Brazil and the Antarctic; others spend their whole life within 50 km of the colony where they were born. However, we know far more about some species than others. The distribution and size of many breeding colonies of the nocturnal petrels are still a mystery but, at the other extreme, every Gannet colony in the world has been counted several times within the last 80 years. Everyone is familiar with the gulls which are a noisy and sometimes troublesome addition to city life, nesting untidily on buildings and thronging rubbish tips and reservoirs. Fewer people may be aware of the truly marine birds which gather in vast numbers to nest in remote parts of Britain and Ireland. In June, seabird colonies on the cliffs, sandy spits, rocky islets and coastal moorland around these shores teem with birds and echo to their cries.

Britain and Ireland have about 2–2.5 million auks and 2.5–3 million pairs of other species (Appendix I). Twenty-four different seabirds nest regularly, out of a total of about 275 species worldwide. Guillemots, Fulmars and Kittiwakes are the most numerous, and the Roseate Tern the rarest. In terms of international status, this seabird community is collectively of considerable importance. However, despite the enormous numbers of seabirds in Britain and Ireland, they are outnumbered by some of the commoner species of land birds. For example, seven million pairs of Blackbirds have been estimated to breed in Britain and Ireland (Sharrock 1976).

WHAT IS A SEABIRD?

Seabirds are a group of birds which depend mainly on the sea beyond the tide-line for their food. The group includes some species which often breed and feed inland, such as the Black-headed and Common Gulls, but excludes other species which spend only part of the year at sea (e.g. divers, seaducks, phalaropes). The adaptations to life at sea summarised in this chapter address the species which live in Britain and Ireland; for a fuller, worldwide appraisal see, for example, Nelson (1980). Although seabirds share a broadly common lifestyle, they are evolutionarily quite diverse. The species which breed regularly in Britain and Ireland belong to seven taxonomic Families:

Procellariidae—four species of petrel (Fulmar, Manx Shearwater, Storm Petrel, Leach’s Petrel)

Phalacrocoracidae—two species of cormorant (Cormorant, Shag)

Sulidae—one species (Gannet)

Stercorariidae—two species of skua (Great Skua, Arctic Skua)

Laridae—six species of gull (Herring Gull, Common Gull, Black-headed Gull, Lesser Black-backed Gull, Great Black-backed Gull, Kittiwake)

Sternidae—five species of tern (Sandwich Tern, Roseate Tern, Common Tern, Arctic Tern, Little Tern)

Alcidae—four species of auk (Guillemot, Razorbill, Black Guillemot, Puffin)

Like many water birds, seabirds have webbed feet and a well-developed preen gland situated at the base of the tail, from which the bird secretes a waterproof oil which is smeared onto feathers during preening. A seabird is able to rid its body of excess salt which it cannot help accumulating through ingesting seawater and food. A highly concentrated salt solution is secreted from the salt gland in the nasal passages and trickles out of the bird’s nostrils. It can sometimes be seen dripping off the tip of the bill.

Seabirds feed generally on fish and marine invertebrates which are high in energy in relation to volume and weight. This diet is a necessary adaptation for a lifestyle which requires long periods of flying when bulky or heavy food in the digestive system would be a disadvantage. Thus no seabird relies upon a diet entirely of plants since this would require the consumption and digestion of bulk food.

Many seabirds spend most of their life in the air and their wings are adapted to the type of flight most often used. The petrels are specialised for a flap-and-glide flight-style and the gliding component approaches perfection in their close relatives, the albatrosses. The Fulmar, for example, has long narrow wings which can carry it for great distances using the winds at the sea’s surface without the need for energetic, flapping flight. Among seabirds, petrels perform some of the longest migrations and are also able to forage far from the colonies during the breeding season. The Gannet and terns have long, pointed wings also adapted for long-distance flight and providing the terns with aerial manoeuvrability. Both the Gannet and terns plunge-dive after fish and undertake long migrations. By comparison, the cormorants, which dive from the surface and swim underwater, have shorter, rounded wings. The auks’ short, stiff wings are the smallest in relation to body size of any existing northern hemisphere seabird group, and are adapted for high-speed propulsion underwater, the feet and short stiff tail serving as rudders and stabilisers.

Petrels, which forage over huge areas of sea, are unusual amongst birds in that they have a finely developed sense of smell. This enables them to detect from afar the presence of oily foods, such as offal or the oil from surface shoals of fish. The habit has been turned to advantage by birdwatchers who throw loads of “chum” (mixtures of fish-oil and offal) from boats at sea in order to attract rare petrels. Petrels are also unique in their ability to store oils in their stomachs. This gives them a slowly digestible food source which provides more energy for its weight than body fat does. It is a vital adaptation for long-distance foraging in adults and for helping chicks to survive periods without food.

The Gannet is a plunge-diving specialist, with thickened bone in the front of its skull which absorbs the impact of plummeting into the water. Air trapped in the thick body-plumage provides buoyancy to help the bird to surface after a dive. Birds which dive underwater from the surface have the opposite problem. Cormorants have to overcome their natural buoyancy in order to reach food on the bottom of the sea. Accordingly, they have a poorly developed preen gland so that their plumage is barely waterproof and unable to hold much air. After a diving session, the birds need to dry their waterlogged feathers and, to this end, are often seen resting on a prominent perch with outspread wings.

For underwater swimming, cormorants—like auks—have a streamlined body and dense plumage which traps little air. The two largest skeletal parts, the breast bone and pelvic girdle, are relatively narrower and longer than in other seabirds, lending the body a torpedo-like shape. The cormorants’ large feet and particularly sturdy legs are set far back on the body and are used as paddles; the wings are kept folded underwater.

SEABIRD FEEDING AND BREEDING

Seabirds feed in many different ways and are able to exploit most marine sources of food as well as some of those on land (e.g. Ashmole and Ashmole 1967). Some seabirds feed mainly at night when organisms which are out of reach in deep water during the day migrate to the surface. The gulls are particularly opportunistic when foraging and adapt their diet accordingly. They are equally at home feeding behind fishing boats at sea, on estuarine mudflats and rocky shores, inland behind a plough or on a rubbish tip, and by hawking for aerial insect swarms.

The petrels and the Kittiwake take food from the surface and from shallow depths; the smaller species do so whilst still in flight. The Storm Petrel and Leach’s Petrel are particularly adept at flitting just over the waves picking up planktonic food or scraps of offal. The largest petrel, the Fulmar, usually flops down onto the surface and plunges its head underwater to grasp a food item. To reach another patch of food, it often runs over the surface of the water, flapping its wings rather than bothering to fly.

Other seabirds exploit generally deeper water layers. All five species of terns hover and plunge head first after fish and crustaceans. The momentum of the dive brings them into contact with their prey without the need for underwater swimming. The depth of the dive depends on body weight so that the Little Tern, for example, feeds largely in surface waters. The Gannet also fishes by plunge-diving and its dives are often higher and more precipitous than those of the terns. Both Gannets and Manx Shearwaters can, if necessary, swim underwater and accordingly they feed at greater depths than any of the terns.

The Cormorant, Shag and the four species of auks dive from the surface and descend into comparatively deep water (see species chapters) or to the bottom of the sea to collect food. The gulls and skuas are opportunistic feeders. They feed on the surface of the sea and just below it, by stealing from other birds, by scavenging behind fishing boats and on land, or by direct predation of other seabirds’ eggs and chicks, occasionally killing even adults. Among the gulls, the Kittiwake feeds furthest offshore, the large species forage mainly in inshore waters and on rocky shores, whilst the Common and Black-headed Gulls are estuarine and inland feeders. The skuas feed mainly by fishing and scavenging at sea, by predation on other seabirds and especially by stealing food from them.

Seabirds can survive and breed only if they are able to find the right kind of food, of a suitable size and at an accessible depth. The continental shelf off northwest Europe provides particularly favourable conditions for fish, supporting both commercial fisheries and a rich seabird community. However, food for seabirds is not evenly distributed over the shelf. Instead, many invertebrates and fish (or their larval and juvenile forms) are concentrated at spawning or feeding grounds and at distinct boundaries where different water masses meet (Cushing 1982). For example, water depth and bottom sediments in the English Channel, Irish Sea, North Sea and the firths of the Scottish coast are such as to provide ideal spawning grounds for many commercial fish stocks, including important seabird foods such as Herring and sandeels. Until oceanic change and fishing pressure reduced their numbers, large shoals of Herring used to feed in the North Sea where their distribution mirrored that of their chief plankton prey Calanus (e.g. Cushing 1952). The North Sea still supports a large commercial fishery for a range of whitefish and flatfish.

The boundary between water of contrasting temperature or salinity is known as a front and these areas can be particularly attractive to fish. Deep water layers of the sea tend to be colder and more saline than surface waters, which are warmed by the sun and diluted by fresh water running off the land. In shallow water near land, the wind and tides mix these layers together. The fronts which occur most commonly around Britain and Ireland are those formed at the boundary between the layered offshore and mixed inshore water masses. These fronts are typified by an abundance of nutrients brought up from the deep water into the sunlit surface waters where phytoplankton can make use of them, the resulting increase in production of plankton attracting fish and seabirds (Brown 1980). The productivity and scale of the front depends on the strength of the forces mixing the water masses together. They may be relatively constant, or may change, for example, seasonally or tidally. The number of birds at a front seems to depend on its strength and persistence, with the weak or intermittent fronts less likely to attract seabirds (Schneider et al. 1987).

Breeding seabirds have two main requirements. They need a supply of food near enough to the breeding colony to be within their foraging range and a safe place with suitable habitat in which to nest. They are able to use a wide variety of different nesting habitats from urban rooftops to the most isolated and inhospitable offshore islands, from cliffs of over 300 metres to low-lying shingle banks which change shape, or even disappear, with each ...

Table of contents

• Cover
• Copyright page
• Contents
• List of figures
• List of plates
• List of tables
• Foreword
• Introduction
• Part 1 Seabird Biology and the Seabird Colony Register
• Seabirds and their Breeding Populations
• The Reasons for Change in Seabird Numbers
• The Methods Used for the Collection and Analysis of Seabird Counts
• Part 2 The Species Accounts
• Fulmar: Fulmarus glacialis
• Manx Shearwater: Puffinus puffinus
• Storm Petrel: Hydrobates pelagicus
• Leach’s Petrel: Oceanodroma leucorhoa
• Gannet: Sula bassana
• Cormorant: Phalacrocorax carbo
• Shag: Phalacrocorax aristotelis
• Arctic Skua: Stercorarius parasiticus
• Great Skua: Catharacta skua
• Black-headed Gull: Larus ridibundus
• Common Gull: Larus canus
• Lesser Black-backed Gull: Larus fuscus
• Herring Gull: Larus argentatus
• Great Black-backed Gull: Larus marinus
• Kittiwake: Rissa tridactyla
• Sandwich Tern: Sterna sandvicensis
• Roseate Tern: Sterna dougallii
• Common Tern: Sterna hirundo
• Arctic Tern: Sterna paradisaea
• Little Tern: Sterna albifrons
• Guillemot: Uria aalge
• Razorbill: Alca torda
• Black Guillemot: Cepphus grylle
• Puffin: Fratercula arctica
• Rare Breeders
• Appendix I
• Appendix II
• Appendix III
• Appendix IV
• Appendix V
• References