Open Access

A review of Ireland's waterbirds, with emphasis on wintering migrants and reference to H5N1 avian influenza

  • O Crowe1,
  • J Wilson2,
  • I Aznar3 and
  • SJ More3
Irish Veterinary JournalThe official journal of Veterinary Ireland, the representative body for the veterinary profession in Ireland200962:800

DOI: 10.1186/2046-0481-62-12-800

Published: 1 December 2009

Abstract

Ireland is characterised by its diversity and large abundance of wetlands, making it attractive to a wide variety of waterbirds throughout the year. This paper presents an overview of Ireland's waterbirds, including ecological factors relevant to the potential introduction, maintenance, transmission and spread of infectious agents, including the H5N1 avian influenza virus, in Ireland. Particular emphasis is placed on five groups of wintering migrants (dabbling and sieving wildfowl, grazing wildfowl, diving wildfowl, waders and gulls), noting that the H5N1 avian influenza virus has mainly been isolated from this subset of waterbirds. Ireland's wetlands are visited during the spring and summer months by hundreds of thousands of waterbirds which come to breed, predominantly from southern latitudes, and during the autumn and winter by waterbirds which come from a variety of origins (predominantly northern latitudes), and which are widely distributed and often congregate in mixed-species flocks. The distribution, feeding habits and social interactions of the five groups of wintering migrants are considered in detail. Throughout Ireland, there is interaction between different waterbird populations (breeding migrants, the wintering migrants and resident waterbird populations). There is also a regular and complex pattern of movement between feeding and roosting areas, and between wetlands and farmland. These interactions are likely to facilitate the rapid transmission and spread of the H5N1 avian influenza virus, if it were present in Ireland.

Keywords

East Atlantic flyway epidemiology H5N1 Ireland waterbirds winter migration

Introduction

The wetlands of northwest Europe are internationally important for millions of waterbirds, both resident and migratory. During summer months, waterbirds migrate to these wetlands to breed, whereas in winter, northern and boreal-nesting species migrate to these wetlands, either to overwinter or on passage to wintering grounds further south. Other waterbirds are year-round residents. Ireland plays a critical role in the ecology of these waterbirds, given its strategic location (on a major flyway), its mild (generally ice-free) climate and abundance of wetlands [2]. An understanding of waterbird ecology is important to disease epidemiology, noting the role of these birds in the maintenance and spread of low pathogenic avian influenza (LPAI) [17]. As yet, however, the role of waterbirds in the epidemiology of highly pathogenic avian influenza (HPAI) H5N1 of Asian lineage remains unclear and developing. The current outbreak of HPAI H5N1 was first detected in China in 1996 [18], and has subsequently appeared in many countries in Asia, Europe and Africa [12]. Ongoing studies are highlighting substantial differences in the behaviour of HPAI H5N1 and previous AI viruses, suggesting that the H5N1 virus is in rapid evolution [19]. These differences include direct transmission of H5N1 viruses from birds to humans, transmission predominantly via the respiratory route, increased thermal stability and varying pathogenicity in waterfowl.

Within the European Union, co-ordinated measures have been developed to prevent and control avian influenza in poultry. Influenza surveillance in both poultry and wild birds has been increased, and import bans are placed on susceptible imports from third countries with H5N1 outbreaks [9]. Ireland has developed a range of relevant measures, including a contingency plan and ongoing risk assessments [5].

Detailed information has recently been prepared on Ireland's waterbirds [2]. As yet, however, a review of this material has not been prepared, nor is information readily available of ecological factors that might be relevant to the introduction, maintenance, transmission and spread of infectious agents, such as the H5N1 avian influenza virus, between and within waterbird populations in Ireland. This paper seeks to address this issue. Particular emphasis is placed on five groups of wintering migrants (dabbling and sieving wildfowl, grazing wildfowl, diving wildfowl, waders and gulls), noting that the H5N1 avian influenza virus has mainly been isolated from this subset of the broader waterbird population [6, 7].

Ireland'swaterbirds

Overview

Ireland is characterised by a diverse and large abundance of wetlands (Figure 1), making it attractive to a wide variety of waterbirds throughout the year. Wetlands are defined by the Ramsar Convention as areas of water, marsh, fen or peatland, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including marine waters, the depth of which at low tide does not exceed six metres [13].
Figure 1

Ireland's wetlands, including rivers streams, reservoirs, ponds, lakes and canals.

Waterbirds are defined as all species which are ecologically dependent on such wetlands, and some 86 species are regularly recorded in Ireland (Table 1), many of which (34 species) occur year-round. However, very few of these are entirely resident. Few remain in Ireland throughout the year, with no immigration or emigration (the Mute Swan Cygnus olor is one.) Rather, the majority of the waterbirds which occur in Ireland are migratory, and are part of the East Atlantic Flyway (Figure 2), delimited by long-term studies of marked (ringed) birds [20]. This is a generalised picture of the distribution and movements of migratory waterbirds and one which has particularly been derived from studies of waders. Details of international flyway delineation, as shown in Figure 2, are less applicable to other waterbird taxa (see [1]).
Table 1

Waterbird species occurring in Ireland

Species

 

Occurrence in Ireland

Divers and grebes

Red-throated Diver

Gavia stellata

Wintering - September to April, small numbers (<10 pr) breed in Donegal

Black-throated Diver

Gavia arctica

Wintering - September to April (Scarce)

Great Northern Diver

Gavia immer

Wintering - September to April

Little Grebe

Tachybaptus ruficollis

Year-round

Great Crested Grebe

Podiceps cristatus

Year-round, numbers increase during the winter due to immigrating birds

Shearwaters and Petrels

Manx Shearwater

Puffinus puffinus

Breeding - March to August

European Storm-petrel

Hydrobates pelagicus

Breeding - March to August

Leach's Storm-petrel

Oceanodroma leucorhoa

Breeding - March to August and passage September to October

Fulmar

Fulmarus glacialis

Year-round

Gannet

Gannet

Morus bassana

Year-round

Cormorants

European Shag

Phalacrocorax aristotelis

Year-round, few seen inshore outside the breeding season

Cormorant

Phalacrocorax carbo

Year-round, some immigration during the winter

Herons

Grey Heron

Ardea cinerea

Year-round

Little Egret

Egretta garzetta

Year-round

Wildfowl (swans, geese and ducks)

Mute Swan

Cygnus olor

Year-round

Bewick's Swan

Cygnus columbianus

Wintering - October to April

Whooper Swan

Cygnus cygnus

Wintering - October to April

Greenland White-fronted Goose

Anser anser flavirostris

Wintering - October to April

Greylag Goose

Anser anser

Year-round (feral birds), augmented by wintering birds- October to April

Canada Goose

Branta canadensis

Year-round (feral birds)

Barnacle Goose

Branta leucopsis

Wintering - October to April

Light-bellied Brent Goose

Branta branta hrota

Wintering - October to April

Shelduck

Tadorna tadorna

Wintering - September to April

Wigeon

Anas penelope

Wintering - September to April

Gadwall

Anas strepera

Year-round, augmented by wintering birds - October to April (scarce)

Teal

Anas crecca

Year-round, augmented by wintering birds - September to April

Mallard

Anas platyrhynchos

Year-round, augmented by wintering birds - September to April

Pintail

Anas acuta

Wintering - September to April

Shoveler

Anas clypeata

Wintering - September to April

Pochard

Aythya ferina

Predominantly wintering - September to April, small numbers breed

Tufted Duck

Aythya fuligula

Predominantly wintering - September to April, small numbers breed

Scaup

Anas marila

Wintering - September to April

Eider

Somateria mollissima

Wintering - September to April

Long-tailed Duck

Clangula hyemalis

Wintering - September to April

Common Scoter

Melanitta nigra

Predominantly wintering - September to April, small numbers breed

Goldeneye

Bucephala clangula

Wintering - September to April

Red-breasted Merganser

Mergus serrator

Wintering - September to April, small numbers breed in West

Goosander

Mergus merganser

Year-round, small numbers breed (scarce)

Ruddy Duck

Oxyura jamaicensis

Year-round (feral birds)

Rail & Coots

Water Rail

Rallus aquaticus

Year-round

Moorhen

Gallinula chloropus

Year-round, augmented by wintering birds - September to April

Coot

Fulica atra

Year-round, augmented by wintering birds - September to April

Waders

Oystercatcher

Haematopus ostralegus

Year-round, augmented by wintering birds - September to April

Ringed Plover

Charadrius hiaticula

Year-round, augmented by wintering birds - September to April

Golden Plover

Pluvialis apricaria

Wintering birds - September to April, breeding birds - April to September, some overlap in populations

Grey Plover

Pluvialis squatarola

Wintering birds - September to April

Lapwing

Vanellus vanellus

Wintering birds - September to April, breeding birds - April to September, some overlap in populations

Knot

Calidris canutus

Wintering birds - September to April

Sanderling

Calidris alba

Wintering birds - September to April

Little Stint

Calidris minuta

Passage birds - August to October (scarce)

Curlew Sandpiper

Calidris ferruginea

Passage birds - August to October (scarce)

Purple Sandpiper

Calidris maritima

Wintering birds - September to April

Dunlin

Calidris alpina

Wintering birds - September to April, small numbers breed in west

Ruff

Philomachus pugnax

Spring and autumn passage (scarce)

Jack Snipe

Lymnocryptes minimus

Wintering birds - September to April

Snipe

Gallinago gallinago

Year round, augmented by wintering birds - September to April

Woodcock

Scolopax rusticola

Year round, augmented by wintering birds - September to April

Black-tailed Godwit

Limosa limosa

Wintering birds - August to April

Bar-tailed Godwit

Limosa lapponica

Wintering birds - September to April

Whimbrel

Numenius phaeopus

Passage birds - April to September

Curlew

Numenius arquata

Year round, augmented by wintering birds - September to April

Redshank

Tringa totanus

Year round, augmented by wintering birds - September to April

Greenshank

Tringa nebularia

Wintering birds - September to April

Common Sandpiper

Actitus hypoleucos

Summer visitor - breeding birds present April to September

Turnstone

Arenaria interpres

Wintering birds - September to April

Skuas, Gulls & Terns

Great Skua

Stercorarius skua

Passage birds - April to October

Arctic Skua

S. parasiticus

Passage birds - April to October

Mediterranean Gull

Larus melanocephalus

Year-round, very small breeding numbers (scarce)

Black-headed Gull

Larus ridibundus

Year-round, some local migration to/from breeding areas

Common Gull

Larus canus

Year-round, some local migration to/from breeding areas in west.

Lesser Black-backed Gull

Larus fuscus

Occur during the breeding season, March/April to August/September

Herring Gull

Larus argentatus

Year-round, some local migration

Great Black-backed Gull

Larus marinus

Year-round, some local migration

Little Gull

Larus minutus

Wintering birds - September to April (scarce)

Figure 2

Wader flyways. Source: International Wader Study Group.

Migratory flyways

The concept of a flyway is based on the migratory behaviour of birds. Flyways in northwestern Europe are typically north-south in orientation, with migratory birds moving to warmer clines during the autumn, and then returning during the spring to more northerly breeding areas. Many species migrate along well-defined routes, and consistently use the same sites as stop-over and/or wintering sites.

Some eight general migratory flyways have been defined [4]. The East Atlantic Flyway (Figure 2) extends from east Canada, across northern Europe to west Siberia in the north, and also south along the east Atlantic shores of northwest Europe and west Africa. Most migratory waterbird species which occur in Ireland during the winter have originated from one or more of the following areas:

  • North/northwest: Canada, Greenland and Iceland;

  • Northeast: the Netherlands, Denmark, Germany, Fennoscandia, the Baltic, western Russia (west of the Ural Mountains);

  • Far-eastern: Russia east of the Ural Mountains and Siberia; and,

  • Central/east Europe: Europe east of and including Poland, Czech Republic and Austria.

The breeding origins of waterbird species occurring in Ireland during the winter are summarised in Table 2. Most migratory waterbird species which breed in Ireland winter further south, in Continental Europe or west Africa (i.e., they belong to the same principal flyway).
Table 2

Flyway origins of species occurring in Ireland outside the breeding season

Species

Breeding1

Wintering1

Flyway estimate2

All-Ireland estimate3

Red-throated Diver

Gavia stellata

Arctic/boreal W Eurasia, Greenland

Europe, Greenland

300,000

1,025+

Black-throated Diver

Gavia arctica

N Europe and W Siberia

NW Europe, Mediterranean, Black and Caspian Seas

375,000

<1,000

Great Northern Diver

Gavia immer

N America, Greenland, Iceland

NW Europe

5,000

<1,000

Little Grebe

Tachybaptus ruficollis

Europe, NW Africa

Europe, NW Africa

340,000

2,345+

Great Crested Grebe

Podiceps cristatus

NW Europe

NW Europe

475,000

5,385+

Great Cormorant

Phalacrocorax carbo

NW Europe

NW Europe

120,000

13,710+

Grey Heron

Ardea cinerea

W Europe, NW Africa

W Europe, NW Africa

274,500

2,960+

Little Egret

Egretta garzetta

Ireland, UK, Continent, N Africa

Ireland, UK, Continent, N Africa

134,000

<1,000

Mute Swan

Cygnus olor

Irelanda

Irelanda

11,440a

11,440

Bewick's Swan

Cygnus columbianus bewickii

Arctic N Russia

NW Europe

29,000

380

Whooper Swan

Cygnus cygnus

Iceland

Ireland, UK, Iceland

20,900

12,730

Greenland White- fronted Goose

Anser albifrons flavirostris

W Greenland

Scotland, Ireland

33,000

11,340

Greylag Goose

Anser anser

Iceland

UK, Ireland

89,100

5,030b

Canada Goose

Branta canadensis

Ireland

Ireland

1,050

1,050

Barnacle Goose

Branta leucopsis

Greenland

Scotland, Ireland

54,100

9,035

Light-bellied Brent Goose

Branta bernicla hrota

Canada, Greenland

Ireland

21,750

30,000

Common Shelduck

Tadorna tadorna

NW Europe

NW Europe

300,000

14,610

Eurasian Wigeon

Anas penelope

W Siberia, NW and NE Europe

NW Europe

1,500,000

82,370+

Gadwall

Anas strepera

NW Europe

W Europe

60,000

630

Eurasian Teal

Anas crecca

N and NW Europe

NW Europe

400,000

45,010+

Mallard

Anas platyrhynchos

N Europe

NW Europe

4,500,000

38,250+

Northern Pintail

Anas acuta

N Europe, W Siberia

NW Europe

60,000

1,235

Northern Shoveler

Anas clypeata

N, NW, Central Europe

NW, Central Europe

40,000

2,545

Common Pochard

Aythya ferina

Russia, NE and NW Europe

NE and NW Europe

350,000

37,780

Tufted Duck

Aythya fuligula

N and NW Europe

NW Europe

1,200,000

36,610

Greater Scaup

Aythya marila

W Siberia, N Europe

W Europe

310,000

4,430

Common Eider

Somateria mollissima

see c below

see c below

1,548,000c

2,890

Long-tailed Duck

Clangula hyemalis

Iceland and Greenland

N Atlantic

125,000

<1,000

Common Scoter

Melanitta nigra

N and NW Europe, W Siberia

Baltic, E Atlantic

1,600,000

23,190

Common Goldeneye

Bucephala clangula

N, NW and Central Europe

NW and Central Europe

400,000

9,665

Red-breasted Merganser

Mergus serrator

NW and Central Europe, Iceland, E Greenland

NW and Central Europe, Iceland

170,000

3,390

Water Rail

Rallus aquaticus

Iceland

Faeroes, Scotland, Ireland

Unknown

Unknown

Common Moorhen

Gallinula chloropus

Europe and N Africa

Europe and N Africa

3,550,000

Unknown

Common Coot

Fulica atra

E, N, W Europe

NW Europe

1,750,000

33,160

Eurasian Oystercatcher

Haematopus ostralegus

N, C, W Europe

Europe, NW Africa

1,020,000

67,620

Ringed Plover

Charadrius hiaticula

Iceland, N and NW Europe

W Europe, N Africa, Mediterranean

73,000

14,580

European Golden Plover

Pluvialis apricaria

Iceland, the Faeroes, Greenland

Ireland, W Britain, Continent, NW Africa

930,000

166,700+

Grey Plover

Pluvialis squatarola

Arctic Russia, NE Canada

Wadden Sea, Ireland, UK, S and W Africa

247,000

6,315

Northern Lapwing

Vanellus vanellus

Europe

Europe, N Africa

6,750,000

207,700+

Red Knot

Calidris canutus

Canada, Greenland

W Europe

450,000

18,970

Sanderling

Calidris alba

NE Canada, Greenland, Svalbard

E Atlantic, W and S Africa

123,000

6,680

Purple Sandpiper

Calidris maritima

E Atlantic

E Atlantic

75,000

3,330

Dunlin

Calidris alpina

N Scandinavia, Russia, NW Siberia

W Europe, Mediterranean, N Africa

1,330,000

88,480

Jack Snipe

Lymnocryptes minimus

N Russia, S Sweden, N Poland, N Belarus, Baltic

W and S Europe, N and W Africa

>1,000,000

Unknown

Snipe

Gallinago gallinago

N Europe

S and W Europe, W Africa

>2,500,000

Unknown

Woodcock

Scolopax rusticola

Europe

W and S Europe, N Africa

17,500,000

Unknown

Black-tailed Godwit

Limosa limosa

Iceland, the Faeroes

Britain, Ireland, Continent, N Africa

47,000

13,880

Bar-tailed Godwit

Limosa lapponica

N Europe, N Russia

W Europe, NW Africa

120,000

16,280

Whimbrel

Numenius phaeopus

Iceland, the Faeroes, Scotland

W Africa

675,000

Unknown

Eurasian Curlew

Numenius arquata

W, N and Central Europe

W Europe, Mediterranean, NW Africa

850,000

54,650+

Common Redshank

Tringa totanus

see d below

see d below

400,000d

31,090

Common Greenshank

Tringa nebularia

Scotland, Scandinavia

W and SW Europe, NW, W and S Africa

420,000

1,265

Ruddy Turnstone

Arenaria interpres

NE Canada, Greenland

W Europe, NW Africa

150,000

11,810

Mediterranean Gull

Larus melanocephalus

Black Sea, C, S, W Europe

Black Sea, Mediterranean, NW Europe, NW Africa

660,000

<200

Black-headed Gull

Larus ridibundus

Greenland, N and W Europe

S and W Europe

4,250,000

Unknown

Common Gull

Larus canus

Iceland, Ireland, Britain

Europe, N Africa

1,725,000

Unknown

Lesser Black-backed Gull

Larus fuscus

Greenland, Iceland, Ireland, UK, Belgium, France

W Europe, W France

550,000

Unknown

Herring Gull

Larus argentatus

Iceland, Ireland, Britain, NW France, Germany

NW Europe

590,000

Unknown

Great Black-backed Gull

Larus marinus

NW France, Britain, Ireland, Iceland, N Europe

E Atlantic S to Iberia

440,000

Unknown

1From Wernham et al. (2002), also EURING's Migration Mapping Tool http://blx1.bto.org/ai-eu

2From Wetlands International [21].

3Crowe et al. [3].

a The Irish Mute Swan population has been elevated to biogeographic population [21], and the threshold is thus the same as that for all-Ireland.

bAdditional birds from feral population - c. 2,000 individuals.

cThe relevant population on which to base the 1% threshold for Common Eider has been taken as the NW European total which comprises the four populations in this region (Britain/Ireland 73,000, Baltic/Wadden Sea 850,000-1,200,000, Norway/NW Russia 300,000-550,000 and White Sea 20,000-30,000).

dThe relevant populations for Common Redshank include two populations, robusta (breed in Iceland and the Faeroes, winter Britain, Ireland, NW France) and brittanica (breed in Britain and Ireland, winter Britain, Ireland, NW France).

Migrants

a. Breeding migrants

During the spring and summer months, hundreds of thousands of waterbirds come to Ireland to breed, predominantly from southern latitudes. These breeding migrants begin to arrive in Ireland in March, and are present in highest numbers between April and August, with some individuals remaining into early October. These waterbirds breed predominantly around Ireland's coastline, colonising the cliffs and islands. Some also nest inland, on lake islands and along the shorelines of lakes, rivers and streams. The breeding migrants include seabirds (petrels and shearwaters, Gannet Morus bassana, cormorants, gulls, terns and auks), as well as a number of other species/groups such as Red-throated Diver Gavia stellata, grebes, herons, swans, feral geese, ducks, rails, coots, waders, Great Skua, gulls, terns and auks.

b. Wintering migrants

During the autumn and winter, Ireland's mild winter climate, relative to most other European countries, together with its diversity of wetlands, make it attractive over a million northern and boreal-nesting migrant waterbirds. Therefore, most of the wintering, or non-breeding, migrants come from northern latitudes from Canada in the west to Taimyr in arctic Russia in the east. These birds begin to arrive from as early as July, with most occurring in Ireland between September and February, and some individuals remaining into early May. They occur in largest numbers of coastal estuaries, and on a selection of inland lakes, such as Lough Neagh in Northern Ireland and Lough Corrib in Co. Galway. Wintering waterbirds include fewer bird groups (divers, grebes, herons, swans, geese, ducks, rails, coots, waders and gulls), but a much broader diversity of species within these groups compared with breeding.

Residents

Many waterbirds remain in Ireland year-round. These include grebes, cormorants, herons, Mute Swan, some duck species and some gulls. However, as mentioned above, Mute Swan is one of very few examples of an entirely resident species, remaining in Ireland throughout the year with no immigration or emigration. Most other species are joined by additional birds during the autumn and winter months migrating from arctic and boreal-nesting areas (see section on migrants below).

Ireland also supports a number of introduced waterbirds, which remain relatively local throughout the year. These include:

Naturalised introductions: Some wildfowl species have escaped from collections, and are free-living (e.g. Greylag Goose Anser anser and Canada Goose Branta canadensis). While these birds are sedentary, and do not generally move large distances, they cannot be easily distinguished from their wild counterparts;

  • Wildfowl bred for hunting: Large numbers of Mallard Anas platyrhynchos are farmed, and released during the autumn for hunting; and,

  • Domestic wildfowl: This group includes farmyard geese and ducks, and birds from wildfowl collections. Some of the farmyard geese and ducks have interbred with naturalised Greylag and Mallard respectively, and are part of the feral populations described above.

Interactions between populations

Most waterbirds occurring in Ireland come from more than one population. For example, small numbers of Coot (almost 4,000 pairs) breed in Ireland [10], while wintering numbers exceed 33,000 individuals [3]. It is believed that the breeding birds are resident, and that during the winter, Britain and Ireland receive large numbers of immigrating birds from northern Europe, Scandinavia and the Baltic [20]. There are many other similar examples where numbers of resident breeding birds are augmented by winter migrants of the same species, e.g. Great Crested Grebe Podiceps cristatus, Teal Anas crecca, Moorhen Gallinula chloropus and Oystercatcher Haematopus ostralegus (Table 1).

Some breeding seabird species, such as Kittiwake Rissa tridactyla, become oceanic in nature outside the breeding season, when they are seldom recorded in inshore waters. Other species are entirely migratory; the five tern species and Common Sandpiper Actitus hypoleucos occur during the breeding period only, while most of the other wader species occur on passage and/or during the winter. For some species, many distinct groups have been recognised, and discrete populations have been defined, some of which have been split taxonomically into subspecies. For example, there are three populations of Dunlin Calidris alpina which occur in Ireland. Calidris alpina schinzii breeds in Ireland and winters further south in southwest Europe and northwest Africa, C. a. arctica breeds in Greenland and occurs in Ireland during spring and autumn on passage, while C. a. alpina breeds in Scandinavia and Siberia and winters in Ireland [16]. Other species, such as Golden Plover Pluvialis apricaria, breed and winter in Ireland, although the origins of the respective populations differ; Irish-breeding birds move south to winter, while the non-breeding birds come from the population breeding in Iceland and the Faeroes. Further details, including the latin names and the status, of all waterbirds occurring in Ireland are presented in Table 1.

Furthermore, there are periods (between March and early May and later between July and October) when there is considerable mixing of breeding and wintering (non-breeding) migrants (Figure 3). Waterbirds of varying origins regularly occur together. Examples include:
Figure 3

The number of migrant and resident waterbird present in Ireland throughout the year.

  • Close to 10,000 post-breeding terns congregate in Dublin Bay in August and September prior to their southward migration and mix with thousands of migrant geese, waders and gulls arriving from faraway arctic breeding grounds; and,

  • Large numbers of Icelandic-breeding Greylag Geese (roughly 2,000 birds) mix with a resident flock of 800 feral Greylag Geese at Lough Swilly each winter.

Interactions between species

Outside the breeding season, most waterbirds are highly gregarious, and generally assemble in large, and often mixed-species flocks. By doing so, they reduce their risk to predation, as a flock of birds is more likely to detect a predator than a single bird. Flocks of swans and geese often include family parties which tend to remain intact for most of the first year. Although preferred wetlands among different waterbird groups (dabbling ducks, swans etc; see below) is to some extent exclusive, some of the larger wetland complexes include a variety of habitat types, and may support many different groups. For example, Lough Derg in counties Tipperary, Galway and Clare supports significant concentrations of a range of both dabbling and diving wildfowl, as well as waders and gulls. Further, waterbird species tend to gather together in mixed flocks where their favoured food is available.

During the breeding period itself, most species tend to nest either solitarily, with many pairs holding and defending territories (e.g. Lapwing), or in dense colonies, where birds nest in close proximity, usually on cliffs and/or on islands, where they afford greater protection from predation and disturbance. It is not unusual for colonial species to nest in mixed-species colonies.

Ireland'swintering waterbirds

Wintering waterbird surveys

Wintering waterbirds in the Republic of Ireland have been monitored for almost 40 years as part of three main surveys, the Wetlands Enquiry (1971/72-1973/74) [11], the Winter Wetlands Survey (1984/85-1986/87) [15] and the Irish Wetland Bird Survey (I-WeBS 1994/95-present) [2]. In most cases, parallel surveys have been carried out in Northern Ireland, with the most recent, the Wetland Bird Survey (WeBS), in operation since 1993/94. An example of information generated from I-WeBS (specifically, the distribution and abundance of Mallard, Anas platyrhynchos) is presented in Figure 4. These surveys have served to highlight the importance of wetlands in Ireland for wintering waterbirds, and have defined a suite of wetlands which have proven of significance, many of which have since been designated as Special Protection Areas (SPAs) under the EU Birds Directive (EEC/79/409) [8]. To this end, waterbird populations, and the wetlands upon which they rely, continue to be monitored in Ireland through I-WeBS and WeBS.
Figure 4

Distribution and abundance of mallard ( Anas platyrhynchos ) in Ireland. These data were collected by BirdWatch Ireland as part of the Irish Wetland Bird Survey. The counts were conducted over the last 10 years, mainly from September to March. At each sampling site, the circle represents the maximum number of mallard supported, based on the maximum number observed at each of the recording sites on a single occasion during the last 10 years.

Within-winter movements

Many waterbird species are highly site-faithful, returning to the same wintering areas each year. While most species are entirely dependent on wetland habitat, others can be found considerable distances from wetland sites. Swans, geese, and some wader species are regularly seen feeding on grassland and/or stubble, and use wetland habitats only for roosting.

During the course of the winter, there is regular temporal and spatial movement of waterbird species. Along the coast, roosting and feeding is largely dictated by tide. Many swan and goose species feed by day, and often fly large distances (occasionally in excess of 20 km) to their wetland roosts at night. Many duck and wader species feed by night as well as by day. The roosting and feeding locations and patterns of movements of these waterbird species are less defined.

Large-scale movements of waterbirds between wetland sites have been directly related to weather conditions. There is a noticeable decline in some waterbirds during early autumn, particularly resident species such as Little Grebe, Cormorant and Mallard. It is thought that this reflects a shift in range of these species as they begin to disperse from coastal sites to exploit small inland wetlands that are replenished by increasing rainfall levels. In contrast, during cold weather periods, when such small wetlands are more likely to become frozen over, there is reversal of this movement, as species are forced at times to move large distances to larger waterbodies, riverine sites or to coastal sites, all which are less likely to become frozen.

During particularly cold weather periods in Europe, a number of species from northern Europe, and even Britain, are known to move west into Ireland, with its milder climate [14]. It is known that Lapwing and Golden Plover are particularly sensitive to such cold weather periods, when numbers in Ireland have been seen to increase dramatically. In extreme situations, Wigeon and Teal in Britain and Ireland move further south into France and Iberia. Here, it is known that there is some mixing with other populations breeding on the Black and Mediterranean Seas.

Waterbird species

Waterbirds are highly variable in size and shape, and have adopted a wide range of techniques that enable them to successfully exploit the variety of food types provided by wetland habitats. In general, wildfowl species feed by dabbling, sieving, diving (for example, to obtain benthic food items, or to prey on fish) and/or grazing, waders by probing or surface pecking, and gulls by surface pecking. Many species exhibit more than one method of feeding. For example, most dabbling ducks are also capable of diving, but do so for much shorter periods of time. Wintering waterbirds can be divided into three main groups:

  • Wildfowl include swans, geese and ducks, and are also often accompanied by their 'allies', the divers, grebes, rails, as well as Cormorant, Little Egret and Grey Heron;

  • Waders frequent intertidal flats and shallow water, especially along coastal sites. This group comprises oystercatcher, plovers, lapwings, sandpipers, curlews and woodcocks; and,

  • Gulls include the large family of seabirds that occupy a wide variety of wetland and non-wetland habitats. Some gull species have learned to co-exist successfully with man and have thrived in human habitats.

Terns are summer migrants, and mostly occur on passage during the autumn and spring months only, and are very seldom recorded during mid-winter.

Almost 140 waterbird species have been recorded during I-WeBS and WeBS. However, this total includes many vagrant species, which for a variety of reasons have strayed away from their usual flyways. More regularly, some 58 species occur in significant numbers at a variety of sites in Ireland, including 33 wildfowl, 20 wader and five gull species.

a. Dabbling and sieving wildfowl

Distribution and feeding habits
Dabbling is the preferred technique, which has given a large group of duck species their collective name. Dabbling ducks in Ireland include Shelduck, Wigeon, Gadwall, Teal, Mallard, Pintail and Shoveler (Figure 5, 6, 7, 8, 9, 10). This technique is also employed by other species, particularly swans.
Figure 5

(Eurasian) Wigeon ( Anas penelope ).

Figure 6

Gadwell ( Anas strepera ).

Figure 7

Teal ( Anas crecca ).

Figure 8

Mallard ( Anas platyrhynchos ).

Figure 9

(Northern) Pintail ( Anas acuta ).

Figure 10

(Northern) Shoveler ( Anas clypeata ).

Dabbling species feed mostly on seeds or invertebrate prey present on the surface of the water. They access food at slightly greater depths by dipping their heads and necks below the water. The long necks of swans allow them to access deeper food items than the ducks. These species can all extend their reach by up-ending into a vertical position, with their tails pointed in the air. Virtually all species which dabble feed by pecking at individual items. However, the dabbling ducks can also feed by sieving dense concentrations of smaller food, where they suck in water through a slightly open bill, and filter out the food items as the water is then expelled out at the sides. With the exception of Shelduck, which is exclusively coastally distributed, most dabbling species occur on a variety of relatively shallow wetlands, both inland and coastal. Dabbling species are often recorded on the lee shore, where the wind has blown seeds and invertebrates into a relatively narrow band. Temporary flooded fields are particularly attractive, where abundant food items surface from the soil or pasture. Wigeon, Gadwall, Teal and Mallard, in particular, are regularly seen taking advantage of such food resources. They are widely distributed throughout the country.

Social interactions

Dabbling species are regularly recorded feeding together in large, and usually mixed, flocks, each species exploiting different feeding niches. Some species gain added benefit by feeding communally. Shoveler are often recorded feeding closely to the tail of the bird in front. The combined action of paddling feet is particularly effective in stirring up the water and increasing the amount of food brought to the surface. Wigeon regularly feed alongside swans and Coot, and parasitise them as they return to the surface with food. Gadwall have also been recorded feeding on aquatic plants brought to the surface by diving Coot and Goldeneye.

b. Grazing wildfowl

Distribution and feeding habits

Whooper and Bewick's Swans, Greenland White-fronted, Greylag, Barnacle and Light-bellied Brent Geese and Wigeon (Figure 5) are all grazing species. The swans and first two species of goose listed also feed on stubble. Light-bellied Brent Goose and Wigeon spend the autumn and early part of the winter grazing coastal vegetation (particularly Zostera spp. (eelgrass), Enteromorpha spp. and Ulvae sp. [green algae]), and move to feed on grasslands once these coastal supplies have become depleted. Most swans and Wigeon, tend to remain close to water at all times, while the remaining species listed above may be found considerable distances from wetlands. All of these species return to roost by night on wetlands.

Whooper Swan and Wigeon are the most widely distributed species of this group, found grazing next to wetlands in almost all counties. Greenland White-fronted Geese are largely concentrated in Wexford Harbour, in particular on the Slobs, though small numbers also occur at approximately 30 locations elsewhere in the country. The range of migratory (Icelandic) Greylag Goose is also quite restricted, and just six flocks are recognised in the Republic. However, the introduced population of this species is much more widespread, and small numbers occur throughout the country, and throughout the year. Barnacle and Light-bellied Brent Geese remain along the coast throughout the winter period. The former species is distributed mostly on the islands along the west and northwest coast of Ireland, where it feeds predominantly on grasslands. Light-bellied Brent Geese occur on estuaries along the east, south and southwest coasts, and increasingly use coastal grasslands as the winter progresses. Wigeon also feed on grassland, but are restricted to those adjoining wetlands, for security (Figure 11).
Figure 11

Pochard ( Aythya ferina ).

Social interactions

While these species tend to mix while in wetlands (as described in 'Dabbling species' above), the majority tend to assemble in single-species flocks while grazing. This is, most likely, due to differences in range, and also preferred feeding times during the day. However, mixed-species flocks do occur. For example, Wigeon and Light-bellied Brent Goose associate at some sites, though mostly when feeding on coastal vegetation. The diets of Light-bellied Brent Geese and Black-tailed Godwits (see 'Waders' below) are markedly different, though they are often seen feeding together.

c. Diving wildfowl

Distribution and feeding habits
Diving species forage by diving in an area, and seek food either visually, or by touch. They feed on submerged aquatic vegetation, or on animal prey, particularly molluscs and crustaceans and/or fish. The list of diving species in Ireland is quite extensive, and includes the divers, grebes, Cormorant, Pochard, Tufted Duck, Scaup, Eider, Long-tailed Duck, Common Scoter, Goldeneye, and Red-breasted Merganser, Moorhen and Coot. Some of the species listed above are largely marine, and are seldom seen close to shore during the winter. These include the divers, Eider, Long-tailed Duck, Common Scoter and Red-breasted Merganser. All remaining species, with the exception of Scaup, which occurs predominantly along the coast and on a few select inland sites, are widely distributed, especially on large deep wetlands. Great-crested Grebe, Little Grebe, Cormorant and Goldeneye are found on a variety of both inland and coastal wetlands, while Pochard, Tufted Duck and Coot are mostly distributed on inland waterbodies (Figure 11 and Figure 12).
Figure 12

Tufted Duck ( Aythya fuligula ).

Social interactions

The marine species listed above are usually found in discrete groups, and seldom interact with other species present inshore, or on inland wetlands. Inland, Little Grebe, Pochard, Tufted Duck and/or Coot are regularly seen feeding together, despite their different feeding habits. Pochard and Coot are primarily vegetarian, while Little Grebes and Tufted Ducks prey on animal material. Tufted Ducks, in particular, also occasionally feed in shallow waters, often alongside dabbling ducks, particularly on spilt grain.

d. Waders

Distribution and feeding habits

The majority of wader species wintering in Ireland are exclusively coastal throughout the winter period. Here, they prey largely on marine invertebrates present in sandy and muddy substrates of estuaries, open coast and along rocky shoreline.

A number of wader species, namely Oystercatcher, Golden Plover, Lapwing, Jack Snipe, Snipe, Woodcock, Black-tailed Godwit and Curlew, also forage on a variety of soil and surface-feeding invertebrates present in farmland. Oystercatchers and Black-tailed Godwits use mostly coastal grasslands, while Golden Plover, Lapwing, Jack Snipe, Snipe, Woodcock and Curlew are much more widely distributed on a variety of both coastal and inland sites. Both Golden Plover and Lapwing (Figure 13) are also known to forage on grassland sites at night, possibly to avoid Black-headed and Common Gulls (see below).
Figure 13

(Northern) Lapwing ( Vanellus vanellus ).

Social interactions

Waders are highly gregarious, and large numbers occur on coastal sites in Ireland. Inland, Lapwing and Golden Plover flocks often mix, and occasionally include other species, such as Black-headed Gull. Curlew are often also present, but in general tend to form more discrete flocks. Jack Snipe, Snipe and Woodcock are skulking species, are highly elusive, and are less likely to interact with the above species.

e. Gulls

Distribution and feeding habits
Gull species occurring in Ireland are predominantly coastal. Most tend to feed at sea, on fish and on offal discarded from fishing trawlers. Some, such as Herring Gull, scavenge close to human habitation, particularly on rubbish tips. Black-headed and Common Gulls (Figure 14 and 15) also occur on a variety of coastal and inland farmland sites throughout the country, where they forage for earthworms and other soil invertebrates.
Figure 14

Black-headed Gull ( Larus ridibundus ).

Figure 15

Common Gull ( Larus canus ).

Social interactions

The feeding and roosting habits of most gull species wintering in Ireland are broadly similar, and large numbers regularly assemble, particularly on coastal sites throughout the Irish coastline. Additionally, Black-headed Gull and Common Gull occur, often together, on a variety of inland sites. Here, they are associated with wetlands, but also regularly occur on pasture. They are often recorded feeding alongside Golden Plover and Lapwing. Both gull species are known to parasitise Golden Plover and Lapwing.

Conclusion

Ireland is attractive to a wide variety of waterbirds, due to the variety and abundance of wetlands. During the winter, over 1 m waterbirds, comprising mostly migrants from arctic and boreal breeding areas, spend the winter in Ireland, congregating predominantly on wetland sites, and occurring alongside resident waterbirds, often in large mixed-species flocks. There is very substantial mixing of waterbird populations in Ireland, throughout Europe and internationally. Throughout Ireland, there is interaction between different waterbird populations (breeding migrants, the wintering migrants and resident waterbird populations) and species. There is also a regular and complex pattern of movement between feeding and roosting areas, and between wetlands and farmland, which will each also increase the mixing of waterbird species and populations. These interactions are likely to facilitate the rapid transmission and spread of the H5N1 avian influenza virus, if it were present in Ireland.

Declarations

Acknowledgements

I-WeBS is a joint project of BirdWatch Ireland and NPWS, and relies on hundreds of dedicated volunteers and staff of partner organisations.

Authors’ Affiliations

(1)
Birdwatch Ireland, Greystones, Co.
(2)
Department of the Environment, Heritage and Local Government, National Parks and Wildlife Service
(3)
Centre for Veterinary Epidemiology and Risk Analysis, UCD Agriculture, Food Science and Veterinary Medicine, University College Dublin

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Copyright

© The Author(s) 2009