Oystercatchers: from shingle beach to roof-top

After over 150 years of successful exploitation of new breeding areas, there are signs that UK Oystercatchers are experiencing predation problems in the Scottish hills and facing disease-related issues on at least one Welsh estuary.

Oystercatcher parents can fly off in search of food for chicks (Tómas Gunnarsson)

There are Oystercatchers flying over our Norfolk garden with worms to take back to their chicks, travelling up to a mile each way on feeding trips. This commuting behaviour opens up nesting opportunities not available to more conventional wader chicks which find food for themselves, albeit with some coaching. A pair of Oystercatchers can nest on the island of a former gravel pit and move up and down a river valley, to feed in wet grassland, pastures and arable fields, or hatch their chicks on the flat roof of a school and probe for worms on the playing fields. This flexibility has facilitated the spread of the species in England, and may be helping to compensate for declining numbers in the core breeding areas of Scotland.

A 150-year story of expansion

The Oystercatcher is a very distinctive and noisy bird. When pairs move into new areas they get noticed, providing us with a reliable history of their range expansion over the last two centuries. We might not now be surprised to see breeding pairs well inland, in lowland river valleys or on upland sheep pasture, but these were solely coastal breeders until about 1840, when the first pairs were recorded inland of the north Grampian coast. This nesting behaviour spread within Scotland and reached the English side of the Solway in about 1900 and Northumberland twenty years later.  You can read more in The Historical Atlas of Breeding Birds

The left-hand map below, from Bird Atlas 2007-11, illustrates the continuation of the expansion of the Oystercatcher breeding range between 1968-72, when the first few pairs had established themselves on gravel pits in the English Midlands, through to 2008-11, when pairs were well spread through English counties north of the M4. Oystercatchers have continued to colonise new counties; areas occupied over the last forty years are shown as red triangles, with larger ones being more recent.

Maps of Oystercatcher breeding distribution from Bird Atlas 2007-11, which was a joint project between the Briitish Trust for Ornithology, BirdWatch Ireland and the Scottish Ornithologists’ Club

At first glance, all looks well. However, the right-hand map, which shows the change in abundance between the breeding season surveys of 1988-91 and 2008-11, tells a different story. Densities have been increasing in England (pink colours – darker means higher increase) but there have been major declines across much of Scotland (grey colours – darker means bigger drop).

Although there has not been a large drop in the number of wintering Oystercatchers across the whole of the United Kingdom, the declining number of breeding birds in Scotland may well be to blame for the drop in winter Wetland Bird Survey (WeBS) counts for the country.  The graph alongside suggests that Scottish estuary counts are down by nearly 50% since the peak counts of 2002/03.

Breeding Season

Schematic summary of migration of Oystercatchers ringed or recovered in Britain & Ireland, from Time to Fly by Jim Flegg (BTO)

Oystercatchers nest early in Britain and Ireland and most will have been on territory for several weeks by the end of March. At the same time, there can still be big flocks on the coasts. Some of these are young birds, which won’t breed until two or three years of age, but others will be adults that are fattening up in preparation for migrations to countries such as Iceland and Norway.  Oystercatchers breed beyond the Arctic Circle and all the way to the northern tip of both countries, where spring arrives several weeks later than it does in the UK.

Verónica Méndez-Aragon

When choosing a territory, a pair of Oystercatchers will look for food and somewhere to place a nest. The archetypal nest site might be on a beach, where the eggs blend in with the shingle that forms the nest cup, marine invertebrates can be collected from below the high tide-line and earthworms may be available on near-by fields, golf-courses or lawns, but the species has learnt to breed in a wide range of habitats.  When selecting somewhere to lay a clutch, usually of 2 to 3 eggs, a simple stretch of the imagination can turn the image of a beach into a river-bank, a gravel track or a bare patch in a field.

Three Oystercatcher chicks on the roof of The Cotterell Building at the University of Stirling (Ben Darvill / BTO Scotland)

Adding in a third dimension takes Oystercatchers to the flat roofs of buildings, such as those of Stirling University.  Here, all that lies between the fox-free shingle roof and worm-rich lawns below is a vertical drop for the chicks, when they are old enough. High-rise living is an international trait in Oystercatchers; one of the birds ringed in Norfolk by the Wash Wader Ringing Group nested in a window-box in Norway and the practice of roof-nesting is common in the Netherlands.

Oystercatchers look after their chicks for much longer than other wader parents.  Most wader chicks start finding their own insect food from day one, following their parents as they learn to pluck prey from the undersides of plant leaves or from the surface of mud and grass mosaics. Oystercatcher chicks expect a lot of their food to be brought to them. Roof-nesting chicks have no choice other than to wait for dinner to be delivered whilst those on the ground are able to do some of the work themselves.  Youngsters will beg for food for several weeks, even when they can fly, extending the parenting period beyond that which is normal for most waders. The team studying the changing migratory behavior of Icelandic Oystercatcher has found youngsters on mussel beds begging for food in October. (See Migratory Decisions for Icelandic Oystercatchers)

Population Trends

The Oystercatcher is an amber-listed species of conservation concern in the UK, to reflect the importance of these shores to the European breeding population of the species and their reliance outside the breeding season on a small number of estuaries. Although we know that individuals can live for up to 40 years, all is not completely rosy for the species, with predation and food supplies causing problems.

As has been discussed in the recent Moorland Forum, ‘Understanding Predation’ report, declines in Oystercatcher numbers have occurred in some moorland parts of the range and there has been a drop in numbers of 29%* across Scotland as a whole, since the start of the Breeding Bird Survey in 1995.  The English population had increased by 56%* in the same period but this does not compensate for the numerically larger losses from the core northern areas. Local representatives and researchers who contributed to the Moorland Forum report highlighted crows, foxes, buzzards and ravens as the four main predators that threaten the success of breeding Oystercatchers.

(* figures for 1995-2017 updated to -38% Scotland and +47% England)

Coastal food supplies are critical for Oystercatchers in the winter months and for at least the first two summers of a young bird’s life.  While they take a wide range of shellfish and worms, one of the key elements of the diet on many estuaries is cockles. In the past this has brought Oystercatchers into conflict with cockle fisheries. There was a major controversy in the 1970s when, at the behest of cockle fishers and despite the objection of conservationists in the UK and in Norway, permission was given to shoot 10,000 Oystercatchers on the Burry Inlet in South Wales – and embarrassment when cockle numbers continued to decline.

These cockles have died very recently (image from CEFAS report)

The latest cockle problem is affecting both birds and people, again on the Burry Inlet but also in sites as far apart as the Dee, the Wash and the Dutch coast. Mass die-offs of young cockles are happening in most years, caused by three new parasites, first identified in Spain, America and Portugal, the worst of which causes the haplosporidian infection. In the five winters up until 1999/2000, the average peak count for Oystercatcher on the WeBS count on the Burry Inlet was 17,188, dropping in each of the next three periods to reach 12,195 in the five years up until 2014/15. This represents a 30% decline.

With no available treatments and with the parasites spreading into new areas, life could get tough for human cockle-gatherers and flocks of Oystercatchers, which rely on shellfish for their income and survival, respectively. You can read more about these issues in a CEFAS report which focuses on the Burry Inlet.

Summary: caring, spreading, declining and threatened by disease

Unlike most wader parents, Oystercatchers find food to feed to their chicks (Tómas Gunnarsson)

• Cockle die-offs have not as yet had a major, national population-level effect on wintering Oystercatchers but the presence of this threat to Oystercatchers and to other species that feed on young cockles, such as Knot and Turnstone, emphasises the need for continued monitoring and the value of WeBS counts.
• Breeding Oystercatchers are in decline in the uplands and there is agreement that the role of predators in the moorland environment needs to be better understood. The Moorland Forum report mentioned above also focuses on four red-listed species: the near-threatened Curlew (see separate WaderTales blog), Lapwing, Grey Partridge and Black Grouse.
• The Oystercatcher has been successfully expanding its breeding range and making use of new nesting opportunities for at least 150 years. Increases in England are, to some extent, compensating for declines in Scotland.
• The fact that parents collect food for their young opens up breeding opportunities in areas where there are inadequate supplies of food adjacent to safe nesting sites. Clever birds!

This is a modified version of an article that first appeared in Shooting Times & Country magazine.

---

WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research.  Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.

@grahamfappleton