It may still feel like summer, and look like summer, but with the advent of September it is now officially autumn and we can look forward to the usual sights and smells of turning leaves and misty mornings. But for birds it has been ‘autumn’ since at least July; as those that were quick to breed (such as Swifts and Cuckoos) have no reason to stay and those that, alas, failed to breed this year head back to their wintering grounds in the south. Waders from the far north are usually the first to head back south – sometimes as early as late June, livening up the birding scene in the UK as they pit-stop along the way.
But it is now that the main migration really starts to get into gear; large flocks of hirundines (Swallows and Martins) are obvious along the coast and on the headlands small warblers skulk in the bushes waiting for the right moment to cross the channel. This is the time of year that gets increasingly exciting for birders as the more obvious migrants build up in numbers with the chance they are concealing a scarce or even rare migrating bird such as a Wryneck or Ortolan Bunting (several of both have been seen in Sussex recently).
This constant yearly cycling of birds from south to north then north to south again is so familiar and predictable that it could be easy to see it as unremarkable. Yet stopping for a moment to think of this mass movement of tiny bodies the remarkableness of it all cannot fail to induce wonder. Arctic Terns travel from the Arctic to the Antarctic twice in a twelvemonth; that’s a brain-boggling distance which takes in multiple continents – these tiny creatures spend the majority of their life on the move from breeding grounds to wintering grounds seemingly with ease!
Then there’s the Bar-tailed Godwit, a wader that has been recorded migrating from breeding grounds in Alaska to winter in New Zealand – flying non-stop across the Pacific ocean between the two. That is not only the longest non-stop flight of any bird, but the longest journey without stops to feed of any animal. All this just to nest in one place and feed in another.
The familiar Wheatear is impressive too – all individuals of this species winter in Africa, yet their breeding range extends across Asia to the edge of Alaska in North America – these robin-sized birds have to migrate back across the Bering straight, across the entire length of Asia then down half of Africa! All this incredible effort is thanks to the Earth’s seasons, a result of the tilt of the planet, which dictate when flushes of food or suitable temperatures arrive in any area of Earth (except the tropics).
Northern Europe is a great place to breed for birds; there is plenty of nesting habitat, loads of food in spring and summer and less competition and predators than in Africa. Yet in winter there are no invertebrates to eat and the temperatures are too low to survive, it is then that Africa is too tempting to resist with its heat and insects. But why do some birds have to fly SO far between breeding and non-breeding areas? This is largely to do with food availability – terns for example are chasing the fish shoals across the oceans when they go from the top of the world to the bottom. But why can’t those Bar-tailed Godwits breed somewhere in northern Asia rather than North America?
Well the Arctic tundra is a very nice place to breed for a ground-nesting bird; vast, vast quantities of food in the forms of flies and vegetation, near-continuous daylight in summer, very few predators and also an abundance of space. As certain areas of the Arctic get more crammed with breeding birds, a percentage of the population is bound to fly on just a bit further to another part of the Arctic that is less busy. This may go on until one year a Bar-tailed Godwit ends up in Alaska – the problem is that it is hard-wired in its DNA to migrate back to the same place as its ancestors have done since forever – New Zealand. So they are unable to change their wintering grounds, and their young, having been born in Alaska, will now repeat the same ridiculous journey due to instinct and memory.
This may not go on forever though as the genes that control migration in birds has been shown to be flexible if circumstances change – at least in some species. Blackcaps for example used to breed in northern Europe then winter in the Mediterranean basin, some still do, but a population in eastern Europe now migrates westward in autumn to winter in Britain, thanks to our now much milder winter climate. Chiffchaffs and a few other passerine species are now doing the same – flagrantly disobeying their deep-rooted migration instincts to take advantage of a changing world.
One wonders if the world was not tilted at all, so that we had no seasons, whether birds would still move around or stay in one place all the time. It would certainly make birding and conservation a lot easier – if a little less exciting and wondrous. So keep your eyes on the sky, bid ‘bon voyage’ to the departing summer birds and ‘bienvenue’ to the arriving wintering species; for the Earth is still turning ever on.