Off to warmer climes
PUBLISHED: 16:37 13 September 2013 | UPDATED: 16:11 18 September 2013
I remember once watching a buff-breasted sandpiper on a golf course on the Scillies. This is an American bird which had been blown off course (excuse the pun) across the Atlantic in a storm. It had intended to fly from North to South America but had ended up in Cornwall and was lucky to be alive. At the time I wondered what would happen to it, would it ever go back or would it be stuck on this side of the Atlantic for life?
The wonder of migration in birds is so incredible to believe that we don’t have to look very far back (in centuries) to find a time when eminent ornithologists believed that swallows hibernated in the mud at the bottom of reedbeds, where they were seen to roost in the autumn, and that barnacle geese might be born from goose barnacles, often washed ashore in the autumn when the geese arrive in Britain from Iceland. Now that we know a little more about migration these suggestions might seem bizarre but it is still extraordinary to think that the arctic tern, whose migration is the longest of any bird on earth, should benefit from making a round trip of twenty thousand kilometres each year. However as you read this article millions of birds, some tiny, will be setting off on a fantastic journey from or to Britain.
Have you ever wondered how they find their way to where they are going?
Many of the methods which enable birds to navigate accurately are still not fully understood but we are aware of some of the strategies they have at their disposal. The first and most readily appreciated is experience. Birds that have already undergone a migration invariably return to the same place, using the same route. To do this they can use physical features in the landscape such as hills, valleys and coastline. Their map of the area where they winter or where they nest is much more accurate so they will remember hedgerows, woods and in the case of the swallow the barn in which they nested. It is also thought that birds remember the locations on which they stop off on their migration a bit like us remembering the best motorway service stations.
A young bird faced with migrating for the first time needs to depend upon strategies other than experience. It is well documented that young warblers explore their home territory quite widely after leaving the nest. They may travel tens of miles in every direction to get a feel for their wider environment. In so-doing they will probably find good nest sites to return to next year and build up a larger experience map of the place they want to return to. This increases their chances of finding their way ‘home’.
It is thought that birds have a genetic desire to migrate so young birds know what they have to do instinctively. Some young birds have it ‘easy’, for example young geese can tag a ride with adults in a skein so they don’t have to do the navigation themselves. Most youngsters don’t have this good fortune, if we consider a willow warbler born on Bodmin Moor for example, it will have to find its own way south to Africa never having done it before.
Many experiments have been performed on captive birds and some have had interesting results. We know that birds use the sun to help orientate themselves. Even though the sun moves during the day, relative to our position on earth, birds can understand this movement and plan a course accordingly. But what about when it’s cloudy or for birds that migrate at night?
Well at night there are stars and the movement of the stars is consistent, for example the stars all rotate around the North Star which is always fixed and visible (at least from the northern hemisphere). Through releasing migratory birds in a planetarium in which the stars have been rotated around a different point it has been observed that they headed off in the wrong direction. So it appears that birds can make use of the stars and moon when migrating at night.
If it is cloudy then I suspect that birds can still recognise the direction of the sun though they also have another strategy which is possibly the most incredible of all. Birds along with many other creatures can detect the direction of the earth’s magnetic field. Using this it is thought that they might be able to tell whether they are heading parallel to or across the magnetic field. The theory is that tiny crystals of magnetite, located in the birds’ head, are used in this process. This has been tested, again using captive birds, in a space where the magnetic field was changed with an electro magnet and the birds were seen to change the direction of their flight.
All of the above skills must be utilised by the homing pigeon, a bird which is known to us all for its ability to find its way home regardless of when and where it is released. But it isn’t just homing pigeons that possess this ability one amazing example is of a Manx shearwater which was taken from its nest in West Wales and, after flying by jumbo jet, was released in Massachusetts; within a week it was back on its nest.
With that in mind I am hopeful that my buff-breasted sandpiper is now back watching golf in America! n