What Happens During Bird Migration & Vagrancy

Dr Stephen Moss, Global Consultant, NETVUE Birdfy

As autumn winds blow and the weather begins to turn colder, the last migrant birds are embarking on their travels – with some wandering off course – while others are arriving to spend the winter with us.

If there is one word that sums up October – on both sides of the Atlantic Ocean – is it ‘windy’. And this October, the southern and south-eastern states of North America have suffered from unprecedentedly high winds, in the form of Hurricane Milton, which passed through Florida leaving a swathe of devastation in its wake.

This is a terrible human tragedy: the loss of life and the destruction of property must be our key concern at this time. But hurricanes, tropical storms and high winds also have a major effect on birds – especially those songbirds, shorebirds and seabirds currently heading south from Canada and the eastern United States to spend the winter in Central or South America.

Fortunately, many species missed the worst effects of the hurricane: the majority of North American warblers, for example, left their northern breeding grounds in late August or September, and by now are well to the south. Indeed, on my recent trip to Costa Rica, in late September, I encountered plenty of these colourful sprites feeding in the forests alongside their resident cousins. Some will stay put there for the rest of the autumn and winter; others have just stopped off temporarily to feed and boost their energy resources, and will then fly onwards, many hundreds of miles further south.

On my side of the Atlantic, in Britain and Ireland, October is the best month to search for North American songbirds that have been swept across the ocean by the tail-end of those tropical storms and hurricanes.

Back in October 1985, I was fortunate to be on the Isles of Scilly, just off the south-west coast of England, when a number of these rare vagrants turned up, having been blown eastwards by the prevailing westerly winds, to make landfall in Europe.

Despite the unseasonably sunny and very warm weather, I caught up with Northern Parula and Myrtle Warblers, Rose-breasted Grosbeak, Red-eyed Vireo, Bobolink and both Yellow-billed and Black-billed Cuckoos. These are common and familiar breeding species for my American friends, but are all very rare in Britain – indeed, I have never seen any of them here since then!

Oddly, on that visit to the Isles of Scilly I also saw vagrant birds from southern and eastern Europe – including Night Heron, Barred Warbler, Bee-eater and Short-toed Lark – and one, a Booted Warbler, from much further east.

In northern and eastern parts of Britain, such as Shetland, Yorkshire and Norfolk, October is also the peak time for vagrants and scarce migrants from the east. Many of these species breed in the Siberian Taiga – the forested region beneath the Arctic Circle – and overwinter in South-east Asia. So what on earth are they doing in western Europe?

It used to be thought that they had simply ‘got lost’; another theory, known as ‘reverse migration’, suggested that these birds had a faulty compass that sent them in exactly the opposite direction from their intended destination. But this does not explain why some species such as the Yellow-browed Warbler, which breeds in Siberia and normally winters in Thailand, has been turning up in far higher numbers than ever before: changing this attractive species from a rare vagrant into a regular passage migrant, especially along the east coast. In some autumns now, hundreds of them arrive on the east coast of Britain.

Scientists now believe that a large proportion of the Yellow-browed Warbler population has permanently changed its migratory and wintering habits: deliberately heading west and south-west, passing through Britain, and then spending the winter in the Iberian Peninsula (Spain and Portugal) or North Africa.

In recent years, some much rarer species that we never expected to turn up in Britain have done so, albeit in very low numbers. A few years ago, in October 2016, a small warbler was found freshly dead on the Isles of Scilly: DNA analysis showed it to be a Pale-legged Leaf Warbler, the first record for Britain and western Europe. Given that this tiny bird breeds in the north-eastern region of Manchuria, in China, and winters in south-east Asia, this was one of the least expected vagrants ever.

Then, at the end of last month, a small warbler found – fortunately alive and well – at Bempton Cliffs in Yorkshire was also identified as this species by its distinctive call: the very first live record in Britain. Birders from all over the country travelled to Yorkshire to try to see and photograph this little bird – and because it stayed for six days, most were successful.

October sees many millions of birds passing through Britain, Western Europe, Canada and the United States; while resident breeders – including swallows, martins, swifts, flycatchers, warblers and chats – have mostly already left, on their long journey south to overwinter in Africa, Southern Europe, Central and South America. But the good news is that many other species – often larger, more numerous and in spectacular flocks – are arriving from the north, to spend the winter in our more temperate regions.

On both sides of the Atlantic, these include shorebirds – plovers, sandpipers, curlews, godwits and so on – and wildfowl: ducks, geese and swans. These birds have bred in the Arctic and sub-Arctic regions, taking advantage of the very long hours of daylight and abundant food resources during the spring and summer months. However, the winters are almost totally dark, and also marked by heavy snow and ice, making it very hard for these birds to find food and survive till the spring – so they need to migrate.

We might think our winter weather can be cold, but compared to these areas it is positively mild; allowing the birds to feed more easily. They gather together in large flocks on wetlands: creating an impressive spectacle. On coasts, where the waters are tidal, they usually go to roost in tight flocks at high tide (whether this occurs at daytime or nighttime), sleeping for very short spells, and always alert to the danger from predators such as Peregrine falcons and other raptors. Then, as the water drops, they fly out onto the growing areas of mud to feed, until once again the waters rise towards high tide. On inland wetlands birds will also feed by day and by night, though they are more likely to roost and sleep during the hours of darkness.

At some bird reserves in the UK, organisations such as the Wildfowl and Wetlands Trust organize daily bird feeds, usually for wintering swans and geese, but also attracting other wildfowl species. One of the most impressive of these is the later winter afternoon ‘Swan Feed’ at the WWT’s headquarters at Slimbridge, on the River Severn in Gloucestershire: this attracts visitors to see Britain’s smallest member of the family, Bewick’s (also known in North America as Tundra) Swan.

These birds have been studied at Slimbridge for over 60 years, originally by the celebrated conservationist Peter Scott and his daughter Dafila, who realized that each swan’s bill has a unique pattern of black and yellow, meaning that individuals can be given names and observed over their whole lifetimes.

Sadly, in recent years numbers of Bewick’s Swans here (and in the rest of Britain) have fallen dramatically, partly as a result of global population declines, but also because winters to the north and east of Britain are now far milder, so that many swans are staying put in the Netherlands and around the Baltic Sea, where they can still find food.

Migration – whether of birds heading away from us to spend the winter in the south, or arriving from the north to overwinter with us – is a key indicator of what is happening to the world’s birds. Unfortunately at the moment it is revealing declines – sometimes gradual but often sudden and severe – in  many of our commonest and most familiar species.