The mysteries of bird migration

Arctic Terns on Inner Farne.
Arctic Terns on Inner Farne.

Bird migration has fascinated mankind for a very long time, away back in the Bible the seasonal arrival and disappearance of birds was noted.

In early times in this country it was thought that swallows overwintered at the bottom of ponds, they would be seen going in to roost in reed beds on an autumn evening and the next morning they were all gone, people could not conceive that they were flying away to Africa and thought they must have gone under the water and from where they would then re-appear the next spring.

At the March meeting of the Scottish Wildlife Trust in Duns Tom Cadwallender put us all right with his talk on ‘Bird Migration’. Birds which can find sufficient food here throughout the year will remain here and are residents. Some birds are partial migrants, the robin in your garden in winter may be a local bird but it could just as easily be a continental bird which has decided to over winter here, whereas some of our local birds may have moved south for the winter.

Chaffinches and siskins also move around depending on the local food supply in winter. Differential migration is when an entire population of birds move away to find food when none is available here.

The sandwich terns which nest on the Farne Islands and on islands in the Firth of Forth go to the coast of north-west Africa whereas the Arctic terns will go all the way to the Antarctic oceans. One bird ringed on the Farne Islands was recovered in Australia, a round trip of 38,000 kilometers. If they live for 25 to 30 years how many miles must they fly?

Swallows will fly as far south as The Cape in South Africa but many birds such as cuckoos, ospreys, sand martins and most warblers over winter just south of the Sahara. They only fly as far as is necessary to find a reliable food supply as it is expensive in energy and in lives flying so far. It is thought as many as 5,000 million birds move from northern Europe to Africa each autumn.

A typical sedge warbler will weigh 11 grams in the summer, but just prior to migrating it will double its weight with stored fat, as it burns off its fat when migrating it will stop off at staging posts to recoup its weight again.

Some birds think that Britain is the ideal place to overwinter. Fieldfares and redwings fly over the North Sea each autumn in vast numbers, as our winters are so much milder than in Scandinavia. Some other birds only migrate from Scandinavia if there is a lack of food there, waxwings, bramblings and short-eared owls have eruptions, winters when they come here in large numbers to avoid starvation. This winter has been a good one for spotting short-eared Owls, there must be a shortage of voles in Scandinavia and quite a number have been seen hunting on rough ground all along the coast from Holy Island to Edinburgh.

To be continued.