Stephen Moss: Migration Begins

It may be high summer right across the northern hemisphere, but for our birds, autumn has begun and migration is in full swing. Over the next three months or so, more birds will travel across the surface of the globe than at any other time of year – literally billions of them will be on the move. And I’ve already seen some of these global travelers on their way south!

The other day I was down at the coast near my home in Somerset, in the south-west of England, visiting my local patch. I come here roughly once a week – sometimes more often – throughout the year, and am always fascinated by the way that things can change, even within a few short days.

My first surprise was that the numbers of birds are really building up now: gulls, ducks and waders – what my American friends call shorebirds – were mostly absent during June and early July, with counts barely reaching double figures. 

But on this visit, in late July, there were flocks in the hundreds. Black-headed Gulls, which breed further north in the UK, were gathering on the water and mudbanks of the River Parrett estuary; flocks of Oystercatchers, Dunlins and Redshanks, along with thousands of Common Shelducks – some of which have travelled here from the Netherlands to moult. There were small groups of Black-tailed Godwits, which have also travelled from either the Netherlands or further north and east in Europe, and will probably spend the winter in France.

These are all fairly short-distance migrants, breeding to the north, and overwintering either here or just a little way south. Then I saw a flash of white – the tell-tale sign of a flying Northern Wheatear; whose name, incidentally, comes from the Anglo-Saxon for ‘white arse’, because of its snowy-white rump. This bird – an adult, accompanied by a smaller and duller juvenile – may have also come from further north, probably England or Scotland, but perhaps as far away as Scandinavia. 

But they won’t hang around. Once they leave Somerset, on a clear night in a day or two, they will fly across the English Channel, then across France, Spain and the Mediterranean. And they won’t stop there: they’ll then embark on an epic trip across the Saharan Desert, all the way across the Equator to West Africa, where they will spend the winter before returning north next spring.

1. When do birds migrate?

August might seem rather early for autumn (or fall) migration to start – yet in fact some birds left Europe and North America some weeks ago, in late June and July, and will already be well on their way to their wintering grounds in Africa, Central and South America.

Some of these will be birds that breed in the far north, beyond the Arctic Circle, and might have failed to raise young this year – so have cut their losses and headed south early. Others are birds that don’t play any part in raising their young, and so have no need to stick around. The best known of these are male and female Eurasian Cuckoos, which are brood parasites (laying their eggs in the nests of other species). As soon as the males have mated with the females, and the females have laid their quota of eggs, they will head south, meaning that they spend just three months in Europe.

Young birds tend to migrate later than their parents. For adult waders and shorebirds, the peak time for migration is August and early September; the neater-looking juveniles peak in mid-to-late September. September is also the peak month for songbird migration on both sides of the Atlantic – both night fliers (most small birds) and day travellers (swifts, swallows and martins). 

For birders in Britain and Europe, and in the USA and Canada, late August and September are usually the most exciting time of year, as we see the greatest variety and numbers of migrating birds – especially if a spell of bad weather brings them down to earth. 

2. Why do birds migrate?

As one eminent migrations scientist once said, “We should not ask ‘why do birds migrate?’, but rather ask ourselves, ‘why do not all birds migrate?’”

That’s because even though migration brings its own dangers – notably bad weather, lack of food and predation – it is also, on balance, the best choice for those species that have evolved to carry out these amazing long-distance journeys across the globe. Staying put has its own dangers: hard winters, especially when they are accompanied by heavy falls of snow or long spells with temperatures below freezing, are very risky for small birds, which can easily starve to death in a single night.

In practice, of course, different species – especially long-distance migrants – no longer have a choice as to whether or not to migrate. These species have evolved the anatomy and physiology they require to undertake these huge journeys, and even if they wanted to stay put for the winter, their instinct means they are impelled to head off.

There are a few exceptions to this, as birds will sometimes alter their migratory strategy in response to changes in circumstances. Several species of hummingbird are now staying put for the winter in Canada and the northern United States, helped by the provision of energy-rich sugar solution in artificial feeders in backyards. 

On my side of the ocean, Chiffchaffs (a small species of Old World Warbler) are now overwintering in southern Britain instead of migrating to Spain or North Africa, as they used to do when I was a young birder. The reason they are able to do so is that in the past two or three decades winters in Britain have become much milder, with far less ice and snow, so that the tiny insects on which this little bird feeds remain available throughout the autumn and winter, especially in wetland habitats. 

3. How do birds migrate?

Different groups of birds – and different species within each group – have very different migration strategies. As I have already explained, day-flying migrants feed as they travel; while night-flying species – most songbirds – feed at each point on their journey. Those that migrate along the coast – notably shorebirds – time their flights to coincide with the best tides to feed, whether that be daytime or nighttime.

Reed Warblers and Sedge Warblers, small reedbed-dwelling birds of the Old World Warbler family, are very closely related, and both overwinter in West Africa. But while the Reed Warbler takes its time, making short hops from one overnight location to another as it heads south, the Sedge Warbler does the entire journey in just two or three giant leaps. 

But the most extraordinary global journey of all is one made by a large wading bird, the Bar-tailed Godwit. A decade or so ago, shorebird researchers in New Zealand put satellite trackers on a number of Bar-tailed Godwits, to see which route the birds took from their breeding grounds in Alaska and their wintering areas in New Zealand, along the western edge of the Pacific Ocean. To their astonishment, one individual godwit made the entire journey non-stop – flying constantly for five days and nights without landing – a distance of more than 12,000 kilometres (about 7500 miles). 

The bird did so by building up huge fat reserves, its weight rising by over half – before embarking on this epic voyage across half the surface of the globe. Like other migrants, it found its way by using the stars and sun, and also the earth’s magnetic field, to work out its geographical position. How it had the stamina and determination to get so far in a single giant leap is one of the many things that make migration so utterly incredible. 

So over the next month or so, take a good look at the birds in and over your backyard or   garden, or on a bird reserve near your home. You may well spot some of these global travellers as they pass through your very own neck of the woods!