The marvel of migration
BIRDS are not the only creatures that migrate, but certainly the best known and perhaps the most observable.
We humans have long been uplifted by the sight of the first swallows in spring, and noticed their imminent departure in autumn, as they line up on telephone wires.
But why? Why do birds risk their lives twice annually, making epic journeys across seas, across vast, arid deserts, where food isn’t to be found?
Simply, it has survival value. The longer days in the northern hemisphere give them more chance to rear and feed a family, but the winters in these cold latitudes bring shorter days, and lack of food, so off they must go, to the tropics, where insects and fruit are plentiful.
Recently, as we know, there have been changes, wrought by global warming, and some birds are starting to change their habits.
White storks, once almost entirely migratory, now overwinter
in large numbers in Spain, and other species are starting to follow suit.
Egyptian vultures, black storks, pallid swifts, even barn swallows and house martins are getting wise to the fact that winters in the south of Spain are not all that bad.
However, the vast majority of migratory birds continue to follow their annual routine.
How do they know when it is time to fly north in spring?
Experiments have shown that subtle changes in daylength somehow affect their metabolism, but we don’t really know how that works.
How do they navigate?
That’s easier. Experiments in a planetarium with redwing, a nocturnal migrant, have shown that they use the stars as a ‘map’, whilst diurnal migrants seem to follow rivers, coastlines and mountain-ranges.
What is much more puzzling, and is something that no scientist has ever been able to explain to me, is the capability that migrant birds have of ‘dead-reckoning’ – how, for instance, a reed warbler, which was caught and ringed on a tiny patch of marshland in Southern England, could fly away, winter in Central Africa with many more of its kind, and find its way back to the same patch of reeds, for the next four years running.
Somehow, I hope we shall never know. There are many amazing feats of long-distance travel, some of which I’ll describe in another article.