Mar­vels of Mi­gra­tion

Mi­gra­tion is a fas­ci­nat­ing sub­ject, but just which birds mi­grate and why do they do it?

Bird Watching (UK) - - Contents -

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.

Pere­grine Tur­tle Dove

Pic­tured above is Burscough, Lan­cashire. The ar­rival of Whooper Swans is just one of the in­cred­i­ble jour­neys that sig­nals the start of the au­tumn mi­gra­tion sea­son. The UK pro­vides the per­fect shel­tered con­di­tions for birds to find a winter sanc­tu­ary, so over the next few months, Wet­land Cen­tres like WWT Martin Mere be­come ‘avian air­ports’, wel­com­ing tens of thou­sands of mi­gra­tory birds, mainly from the Arc­tic. At the same time, birds that ar­rived in spring to raise their young are head­ing south to over­win­ter in in­sect-rich Africa.

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