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Rare Waders - how do they get here?

Text & images: Eric Dempsey.

American Golden Plover - a high flyer - perhaps these birds are carried to Ireland on the North Atlantic jet streams
While over 60 species of wader have been recorded in Ireland, only eleven breed here on a regular basis. Most are winter visitors from their breeding grounds further north in Iceland, Scandinavia and the Arctic Tundra's. Many more are autumn passage migrants; that is, they are only passing through Ireland on migration. Others may be lost vagrants from North America or Siberia. So we don't usually have the pleasure of seeing waders in full summer plumage. The best we can hope to see are a few birds that have transformed into their 'Arctic Tundra' attire in late spring before they move north. Therefore, we have to concentrate on those returning birds in autumn when they are most difficult to identify and autumn and winter are the best times for wader watching.

Black-winged Stilt - a very rare southern European bird
Every autumn, from late summer into early November, millions of birds are on the move. Ireland becomes a virtual avian airport. Departing on long-haul flights are birds like Swallows journeying to far-off destinations such as southern Africa, while in the arrival lounge are geese, ducks and waders. The transit lounge is also full, with many species simply stopping off in Ireland en-route to other destinations. Most of these transit visitors are waders like Little Stints and Curlews Sandpipers.

Buff-breasted Sandpiper - it is hard not to marvel at the journey they have made from their high Canadian Arctic breeding grounds
Many of these flights have been planned well in advance and the departure and arrival times are well understood. At this stage, the migration routes of so many species are known. Swallows winter in southern Africa, Black-tailed Godwits in Cork. So why would a bird, such as a Buff-breasted Sandpiper that should be wintering on the grasslands of South America turn up in Ireland instead?

Curlew Sandpiper - transit migrant, these birds neither breed or winter in Ireland, but simply transit through Ireland en-route to Africa.
One of the main reasons is in understanding the important difference between spring and autumn migration. In spring, migrating birds are usually adults. They have migration experience. Some may have completed this journey many times over. However, in autumn, the majority of birds migrating are young, inexperienced birds, making this journey for the very first time. They are unfamiliar with the best routes and they simply get lost. This theory is supported by the fact that most 'rare and uncommon birds' that are found each autumn in Ireland are invariably young birds. This seems the most likely and simple explanation. But can this really explain how, for example, an American wader, migrating from Canada to South America, can suddenly turn up on an Irish wetland in autumn. Or does it explain why a Siberian wader migrating to south-east Asia, can turn in Cork? Are these birds simply lost, or are there other factors at play?

Lesser Yellowlegs - research has shown that this species is capable of flying over 2,000 miles without needing to feed.
In the case of birds from North America, weather is perhaps one of the key factors. Lying on the western edge of Europe, Ireland is one of the first landing points for transatlantic vagrants. In some years it is not unusual to encounter five species of North American wader in one day, with the peak period from early September into mid-October. In fact, species such as Pectoral and Buff-breasted Sandpipers occur with such regularity during this period, that they are virtually annual.

The weather systems responsible for transporting such birds to this side of the Atlantic are now better understood. Many North American species migrate to South America down the western Atlantic coast, some undertaking non-stop flights. Leaving from sites in eastern Canada and north-east USA, it is believed that many leave in the wake of a cold front, taking advantage of the north-westerly tail wind.

Problems occur however when such migrants meet slower moving depressions requiring them to fly through the associated bad weather. Many birds, especially young birds, do not successfully negotiate these weather systems and are drifted out across the Atlantic on the westerly winds. Depending on the prevailing meteorological situation, these weather fronts can move very rapidly across the ocean driven by the high altitude jet streams. Birds caught out in these systems can either fight the wind or simply fly with it.

Pectoral Sandpiper - this North American species occurs in Ireland annually.
However, there may be more complicated factors at work. Because most waders are high-altitude migrants, it may not be the low-level weather systems that bring birds across the Atlantic, but westerly airflows in the high altitude jet stream. As a result, vagrants can occur despite the apparent lack of strong westerlies; arriving instead on jet-streams that are undetectable on daily synoptic weather maps: the sort that we consult when we want to check the likelihood of a vagrant being swept across the Atlantic. As some birds travel above the clouds, this may even account for the occurrence of species along the eastern coast of Ireland, with birds not dropping down until they see land.

There is one final consideration to take into account for the occurrence of North American waders. For birds to cross the Atlantic, regardless of the weather systems that may have brought them, they would still need to have enough stored fat reserves to make such a journey in one flight. For example, recent research shows that Greater Yellowlegs (a wader very like our Greenshank) may be only capable of storing enough fat for a maximum journey of 1,650-1,800 miles, compared to the maximum of 2,125 miles for Lesser Yellowlegs (similar to Redshanks). Therefore it is only in exceptional circumstances that Greater Yellowlegs could reach Ireland and may account for the rarity of that species here compared to Lesser Yellowlegs. It is worth noting of course that, regardless how they get here, the majority of North American birds found each autumn are young birds, making the long migration for the first time.

White-rumped Sandpiper - usually found with Dunlin, this high Arctic North American species frequently gets blown across the Atlantic in autumn.
While weather may account for the occurrence of species from North America it can't really offer a useful explanation for birds that occur from the east. Surely a bird beginning its migration from breeding areas in central or northern Siberia can not be blown off course by bad weather and end up in Western Europe? The distances these birds travel to get here are so great that any weather system that may have influenced them initially, would have subsided long before they even reached Eastern Europe. And yet, they still turn up.

Wilson's Phalarope - unlike most other waders, phalaropes are at home swimming in the sea. So if this North America bird got tired, it could simply pitch down and rest on the Atlantic
One of the most exciting theories to emerge is that of 'reverse migration'. It suggests that birds from the east that occur here are actually flying here deliberately. All migratory birds are born with a mini-compass, enabling them to fly in the right direction. However, if the theory of reverse migration is correct, it suggests that some birds are actually born with a slight defect that causes them to fly in the wrong direction. In other words, instead of flying south-east in the autumn, they will fly in the opposite direction, i.e. north-west.

To understand how this can effect migrant birds, it is important to remember that the world is a globe. Therefore if a bird beginning its first migration from western or central Siberia in late summer, it should start flying south-east to its wintering grounds in China. If however it starts heading north-west instead, where might it end up? Using a globe, the answer is simple, it will probably cross over the Urals, move through the Baltic and, by late autumn, may end up in Western Europe. Of course, weather patterns can alter their direction when they reach the Baltic regions or begin crossing the North Sea.

Little Stint - transit migrant, these birds neither breed or winter in Ireland, but simply transit through Ireland en-route to Africa.
So is reverse migration serving an actual purpose? Are these birds 'frontier explorers', seeking out new wintering grounds? And if they are successful, and return to breed in their usual summer range, will their offspring begin to migrate to the new wintering area? The advantage of this would only be known if there was a natural disaster in the usual wintering grounds. In such an event, the fact that these 'explorers' may have found new wintering areas might actually result in the survival of the species.

Short-billed Dowitcher - one of the rarest waders ever seen in Europe, this North American bird associated with godwits when seen in Ireland
Such theories don't have to apply only to far distant migrants. In the last two years the first ever wintering Little Ringer Plovers have been found in Ireland. I found the first bird in December 2003 in Mallow, Cork. The second occurred the following year at Ballycotton, also in Cork. This species breeds as close to Ireland as Great Britain, but the entire northern European population migrates south in winter. So, did these birds come from Central Europe? Did they originally intend to fly south-east to the eastern Mediterranean, but actually flew north-west to Ireland? Here they enjoyed a food-rich environment and relatively mild winters. Both stayed until early spring when, presumably, they returned to Europe to breed. Will their young, inheriting the 'wrong' migration instincts, return to Ireland this winter?

Little Ringed Plover - I was in the presence of a great explorer
Wader migration is a breathtaking natural event. It is one of the greatest spectacles on earth and has fascinated man for thousands of years. When I see American waders in Ireland, I find it hard not to marvel at the journey they have made. The fact that they have survived their transatlantic flight is a testimony to their instincts and resilience.

And when I looked at that Little Ringer Plover in Mallow on that dull and dark December afternoon in 2003, I didn't see a lost soul. No, instead, I felt I was in the presence of a great explorer, pushing out the frontiers for its species.

To find out more on rare birds in Ireland, visit www.birdsireland.com.

© Eric Dempsey


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