Fall Vagrancy

Dr Stephen Moss, Global Consultant, NETVUE Birdfy

As millions of birds set off on their migratory journeys, travelling thousands of miles from the northern to the southern hemisphere, some go astray. How and why they do so, and what happens to them afterwards, is the subject of my September blog.

Imagine being a bird – a small songbird such as Blackpoll or Myrtle Warbler, for example – setting out on your epic journey south for the winter.

This is likely to be your very first migration: most songbirds only live for one or two years, so the vast majority of travellers are birds that hatched earlier this year, in the woods and forests of Canada and the northern states of the USA.

You set off on a cool, clear September evening, taking advantage of cloudless skies and a following wind. Birds such as new world warblers, flycatchers, thrushes and chats migrate after dark, as this enables them to avoid diurnal predators such as hawks. Having flown all night they will usually land just before dawn, and spend the day sleeping and feeding, building up their strength for the next night’s journey.

Which route do they take?

Many migrants head along the eastern seaboard of North America. But some species – especially those that breed in eastern Canada – take a very different route: heading out into the North Atlantic Ocean. This apparently bizarre behavior makes perfect sense of you take a look at a globe. This reveals that this oceanic route is by far most direct and shortest one to their wintering areas in Central and South America.

Yet there is one significant drawback to flying over the ocean. September is a very windy month, and from time to time the tail end of a hurricane will sweep these tiny birds – most of which weigh less than an ounce – out into the ocean.

At this point, these birds face a race against time. If they have only just set off, and the winds are not too strong, they may be able to reorientate and get back to their intended route. For the vast majority, though, the strong winds will prove fatal: they may battle against their fate, desperate to survive, but eventually they will succumb to their fate, and fall exhausted into the sea and drown.

Yet not all will die. Each year, a small number of birds, more by luck than judgement, are swept across the ocean and make landfall on the other side. These birds – now known as vagrants because of their rarity – are an annual highlight for British and Irish birders, who flock to islands and headlands along the west coast, hoping to find these lucky birds.

Key sites include the Isles of Scilly, off the south-west of Lands’ End, Cape Clear island in southwest Ireland, and in recent years, islands off the west and north of Scotland, such as Barra in the Outer Hebrides, Orkney and Shetland. This is due to a northward shift in the jetstream, which forces these lost birds further north, and a major increase in the number of expert birders heading to these windswept sites during the autumn migration season. But North American vagrants can and do turn up almost anywhere, including, on occasion, on garden bird feeders!

Further south, the Atlantic archipelago the Azores also sees mass arrivals of North American vagrants – some of which are so regular that they should more properly be described as ‘scarce migrants’ rather than true rarities. Their arrival here is hardly surprising, given that the Azores are so far out into the ocean, and much further south than Britain and Ireland. 

The most sought-after – and rarest – species on this side of the Atlantic are North American songbirds such as vireos, thrushes, flycatchers and – especially – warblers. But species from many other bird families make the crossing, including shorebirds, ducks, gulls, terns, herons and egrets. Again, many of these are better classified as scarce migrants, with Pectoral and Buff-breasted Sandpipers, Laughing Gull, American Wigeon and many other species annual visitors, sometimes in the dozens.

Events of autumn 2023

The ornithological event of 2023 – and for rarity-hunters one of the most amazing events ever – happened in the last week of September of that year. Tropical Storm Lee, which occurred slightly earlier in the autumn than most such events, produced winds of up to 165 miles per hour in the western part of the North Atlantic Ocean. Its timing coincided more or less exactly with the peak season for migrating songbirds off the eastern seaboard – especially warblers and vireos.

In less than a week, dozens of vagrant birds were found – most of them in the less watched and less famous birding areas of the west coast of Wales and its offshore islands. In the next week or two more than 100 birds of two dozen species were found, including such (British) rarities as Canada, Magnolia and Blackburnian Warblers, each of which had only be seen a handful of times before, and so caused a major series of ‘twitches’ as birders rushed west to connect with these rare and beautiful birds. Less rare species such as Red-eyed Vireo were reported in their dozens! To my North American readers, it might seem odd that birds which Britons can see simply by flying across the Atlantic  are so sought-after here in the UK – but for us, these species are far from commonplace.

What happens next?

Once they’ve seen and ticked off these American vagrants, few birders ever consider what happens next. The sad truth is that, because the prevailing Atlantic weather systems are mostly westerlies, it is highly unlikely – indeed, almost certainly impossible – that they will ever manage to reorient themselves, cross back over the ocean, and find their way back to their intended destination.

Some, however, do manage to buck the trend. In the winter of 1954 to 1955, a Myrtle Warbler was found on a bird table in a garden in Devon. This was not only the very first record of this common and familiar North American species in Europe, but one of the first songbird every recorded on this side of the Atlantic Ocean.

The fate of larger, more robust and long-lived species, is very different – and far more positive. Thirty years ago, while taking part in a ‘bird race’ in Norfolk in May, I was amazed to see a Lesser Yellowlegs (an elegant North American shorebird) in a small, muddy pool at the coastal reserve of Cley-next-the-sea.

It was accompanied by a Wood Sandpiper, a slightly smaller Eurasian wader that winters in Africa and breeds in Scandinavia and northern Europe and Asia. We surmised that the yellowlegs had crossed the Atlantic the autumn before, then headed south to somewhere in Africa, before flying north that spring alongside its Old World cousin.

Other North American vagrants return to the same site year after year – like the Ring-billed Gull which I used to see every January on a playing field in West London, or the Bonaparte’s Gull which has just returned to a marsh in North Kent for the fourth summer in a row! On very rare occasions, they even breed in Britain: a pair of Spotted Sandpipers nested in a remote site in the Scottish Highlands back in 1975, while earlier this year a pair of Blue-winged Teals almost certainly bred at a reserve in Yorkshire.

Vagrancy in North America 

While the east coast of the United States doesn’t get the same kind of vagrants as Europe – mainly because birds are very rarely blown westwards across the Atlantic – places such as Cape May, a seaside town in New Jersey, do get their fair share of unusual visitors. Cape May has justifiably been described as the best place to witness bird migration on the planet; and in both spring and fall the numbers and variety of species are jaw-dropping. I once watched a Lark Sparrow – a species usually found further west in the USA – by the hawk viewing platform there; while after a tropical storm back in September 1999 I witnessed large numbers of Sooty and Bridled Terns, which usually live in tropical seas far to the south, passing the coast. That same year a Yellow-nosed Albatross from the South Atlantic was also seen, though sadly I missed it!

Occasionally, European vagrants do manage to reach the eastern seaboard: almost a century ago, back in 19237, large flocks of Lapwings crossed the Atlantic following a storm. But these birds are few and far between compared with American vagrants reaching Europe.

The future of vagrancy

We are currently experiencing two contrasting factors, one of which is leading to a rise in the numbers and variety of vagrant birds, and the other leading to a fall. Ironically, the climate crisis is at the heart of both.

Climate change is clearly leading to more extreme and frequent unusual weather events, such as the timing and severity of Storm Lee. This in turn are more likely, at least in the short term, to bring vagrants across the ocean.

However, climate change – combined with habitat loss and degradation across much of eastern North America – is also leading to a major reduction in the populations of the breeding birds of that region. So twitchers need to take advantage of these birds while they can – within the next decade or so, they might no longer be making the Atlantic crossing.