Marvels of Migration
Migration is a fascinating subject, but just which birds migrate and why do they do it?
Learn all about bird migration in this special section, brought to you in association with Hawke
In a nutshell, birds migrate for two main reasons – to breed, and to find enough to eat. And as the success of the former is dependent on achieving the latter, in fact it’s for one reason – food. So, the birds that arrive here in spring do so because Britain has the natural food, namely insects, that they need to raise their young, and they depart again in autumn because they have more chance of finding enough food in more southerly, warmer climes, whether around the Mediterranean or in sub-saharan Africa. Other factors can come in to play, as you’ll see, but all essentially come back to food, and survival. Britain’s migrants, then, can be split into several categories, which you can read about here… Full migrants These are the species that, year in, year out, arrive here in spring from further south (Swallows and other hirundines, Swifts, most of our warblers, Cuckoos, and Turtle Doves, to name a few), or that arrive here in autumn from their Arctic breeding grounds, to take advantage of our relatively mild, ice-free winters (predominantly ducks, geese and swans, and many waders). Passage migrants These are the species that pass through Britain on the way to and from their destinations, often stopping off to feed for a few days as they do so. Whimbrels are a good example – the vast majority that we see here are travelling to and from Iceland, where huge numbers breed. In some cases, the numbers of a British breeding bird are swollen temporarily by these passage migrants, Scandinavian Ospreys, for example, pass through before heading further south. Partial migrants Some species are largely sedentary in some areas, but highly migratory elsewhere – for example, Starlings in the UK tend to stay put, but those in eastern and central Europe migrate to escape the worst of the winter weather (many of them coming to Britain). Similarly, Lapwings and even very familiar species, such as Robins, behave this way – our own redbreasts remain all winter, but you may see or hear them squabbling with incomers from further afield. Altitudinal migrants These are upland species that head down to lower altitudes, where it’s warmer and there’s more food, in winter. These can include raptors such as Merlins, Peregrines,
Hen Harriers and Short-eared Owls, passerines such as Meadow Pipits, Sky Larks and Snow Buntings, and waders such as Dunlin and many of the UK’S Curlews. Moult migrants Some birds migrate just before their main moult, seeking out safe areas for this vulnerable period, where they can grow their new feathers undisturbed. Shelducks are probably the best known, with most of the UK’S population heading to the area around Heligoland in late summer and early autumn, but even the much-maligned Canada Goose has started to do this – some southern breeding birds head for Scottish lochs where they can moult. ‘Hidden’ migrants OK, so we just made that term up. But ringing recoveries and other studies have shown that supposedly sedentary species move around within Britain, with Blackbirds from East Anglia, for example, heading for the warmer south-west in winter. Huge flocks of Woodpigeons, too, move considerable distances in response to weather or feeding opportunities, and one Black-tailed Godwit was shown to have been commuting between East Anglia and the north-west almost daily. That’s what we mean by hidden migrants. Vagrants These are birds that end up here because they’ve become diverted from their usual migration route, because of bad weather such as gales, or because the bird itself (usually young and inexperienced) has taken the wrong direction or overshot its intended destination. They can arrive from Europe, and even further east, while in autumn vagrants from the Americas are particularly prominent in westerly parts of Britain, pushed this way by hurricanes and other storms.
A word of warning
Some species fall into more than one of those categories. For example, we have a small breeding population of Black-tailed Godwits, but also get many more as passage migrants.
When do birds migrate?
There’s practically no time of year when there isn’t at least some migration going on, but the main spring migration period starts around the second week of March, when Wheatears and Sand Martins start arriving here, and goes on until late May, when the last Swifts, Spotted Flycatchers and Turtle Doves drop in. Autumn migration can start as early as late June, when Arctic-breeding waders start to return to the UK or pass through, and goes on into November, when House Martins have left and the bulk of winter geese and swans arrive.
How do birds migrate?
That’s the million-dollar question. It’s been suggested that they use the sun, stars and the earth’s magnetic field to navigate, as many young birds do so accurately on their own (young Cuckoos, as the most extreme example, know no adult Cuckoos they could follow), and it’s likely that all three play a part. Some birds do follow others, though, such as skeins of geese. Smell may play a part – studies have shown that homing pigeons use it to navigate. And they also use landmarks and geographical features, especially when they get close to their destinations, and especially in those species that follow a well-defined ‘flyway’ (others may migrate on a much broader front). Ringing recoveries and satellite tagging are starting to uncover these mysteries of how migration happens – see page 12 for more information.