An­i­mal mag­netism be­hind nav­i­ga­tion

Macclesfield Express - - THE LAUGHING BADGER - SEAN WOOD

EV­ERY year birds mi­grate to our shores from as far away as Africa and Rus­sia.

Amaz­ingly, very few birds seem to take a wrong turn on their jour­ney, with some even re­turn­ing to the ex­act same spot ev­ery year.

So with nei­ther road signs nor a com­puter GPS to guide them, how on Earth do birds nav­i­gate from one place to an­other? It turns out that some birds have tiny grains of a mag­netic min­eral called mag­netite in their heads. This min­eral can de­tect the Earth’s mag­netic field.

To test if birds do use mag­netic fields, an ex­per­i­ment in the 1960s took some robins and placed then in cages with ar­ti­fi­cially-cre­ated mag­netic fields. They found that the birds ori­en­tated them­selves ac­cord­ing to the new mag­netic fields and that the new di­rec­tion cor­re­sponded with the route they would have taken had they been mi­grat­ing nat­u­rally. We know that birds fol­low the same mi­gra­tion routes each year, but whether birds learn a ‘map’ of their im­me­di­ate sur­round­ings via re­mem­ber­ing spe­cific land­marks on their jour­ney is hard to prove. How­ever, the hum­ble star­ling has of­fered some in­sight into this is­sue. Just as they were about to start their mi­gra­tion, thou­sands of star­lings were cap­tured in the Nether­lands and then re­leased in Switzer­land.

The ju­ve­nile star­lings set off in the di­rec­tion that would nor­mally have taken them to their win­ter­ing grounds had they still been in the Nether­lands. The adults, how­ever, cor­rected for their new lo­ca­tion and nearly all reached their nor­mal win­ter­ing grounds. It seems that birds with ex­pe­ri­ence of a lo­ca­tion can re-find it and that they must to some ex­tent have ‘map sense’ to know where they are in re­la­tion to their home, based solely on their cur­rent lo­ca­tion.

For birds that mi­grate at night, star po­si­tions are im­por­tant in help­ing them get to where they are go­ing. Birds put in plan­e­tar­i­ums where the star pat­terns had been changed be­came con­fused, while on over­cast nights there tends to be less recorded mi­gra­tion.

That birds use the sun to nav­i­gate has been known for more than 50 years. And it’s thanks, again, to star­lings.

In ex­per­i­ments that changed the sun’s di­rec­tion, it was shown that star­lings cor­rected their flight paths to take ac­count of the sun mov­ing. Lastly, we come to good old-fash­ioned learn­ing. For those species who mi­grate as fam­i­lies (e.g. geese and cranes) it seems that the young learn the routes from their more ex­pe­ri­enced par­ents.

Once learned, younger birds are then able to travel the route suc­cess­fully them­selves.

An­cient books show how peo­ple in early times noted the com­ings and go­ings of birds. In the Iliad, writ­ten in the Eighth Cen­tury BC, the Greek poet Homer de­scribed the Tro­jan army as be­ing ‘Like the cranes which flee from the com­ing win­ter and sud­den rain’. In the Bi­ble, the prophet Jeremiah says: ‘Yea, the stork in the heaven knoweth her ap­pointed times; and the tur­tle (tur­tle dove) and the crane and the swal­low ob­serve the time of their com­ing’ (Jeremiah 8: 7).

Thou­sands of years ago, mi­gra­tory birds were an im­por­tant source of food. No won­der peo­ple no­ticed when they dis­ap­peared! The an­cient Greek sci­en­tist and philoso­pher Aris­to­tle made the first se­ri­ous at­tempt to ex­plain mi­gra­tion. He even spot­ted that birds get fat­ter while mi­grat­ing. But he didn’t get ev­ery­thing right.

For ex­am­ple, he thought that many birds, in­clud­ing swal­lows, storks and kites, hi­ber­nated in trees for win­ter. He even be­lieved in trans­mo­gri­fi­ca­tion, which is the idea that some species mag­i­cally change into oth­ers – such as red­starts be­com­ing robins for win­ter.

Some early ex­pla­na­tions for mi­gra­tion seem pretty bizarre to­day.

When a swal­low was found dead in a reed-bed, peo­ple thought this proved that swal­lows hi­ber­nated in the mud at the bot­tom of ponds. Be­lieve it or not, peo­ple even thought that bar­na­cle geese hatched from bar­na­cles.

Per­haps this was be­cause of the colour of bar­na­cles or be­cause their ‘feet’ (called cirri) look a lit­tle like feath­ers. In fact, bar­na­cle geese mi­grate to Green­land to breed, but it was not un­til 1891 that a Euro­pean sci­en­tist ar­rived there and ac­tu­ally saw one of their nests.

All around the world, peo­ple had their own ideas. The Cree In­di­ans of North Amer­ica thought that small birds, such as war­blers, mi­grated on the backs of larger ones, such as sand­hill-cranes.

The Laugh­ing Badger Gallery, 99 Platt Street, Pad­field, Glos­sop

●● Cranes fol­low­ing their mi­gra­tion route

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