5 Mar 2010

More on Bird Migration

Now let us have something to say about methods of studying bird migration, although I do not wish to use the term "studying" in the sense of regimented bird study by bureaucratic, but by scientific and democratic, organisation. Ordinary casual watching by the week-end visitor to bird haunts provides the bulk of those "records" which not only fascinate the bird-watcher but have contributed much valuable information to the study of bird migration, and many field-reports for the surveys organised by some of the scornful critics of amateur ornithologists. Perhaps it is the element of surprise as to what may turn up next which gives bird migration such a wide appeal. In my particular area the arrival on local waters of the wild duck is of an endless interest. Pochard and Tufted Duck, the two commonest divers, first appear in August, probably coming from British nesting haunts. They are followed by Teal, Shoveller and Scoter in September, the fishing Mergansers reach the estuary, and a few Pintail come to the lakes and estuaries in October. Wigeon, Scaup and Goldeneye appear in Novem¬ber along the marine lakes when the holiday crowds have gone home from the seaside. In December there is a large immigration of Mallard, presumably from North Europe where the cold weather is setting in for the winter. Many Mallard visit the sea off the Lancashire coast by day—and then in January one usually sees a few red-headed young Smews on lake or mere. One day early in October I was astonished at the numbers of Black Duck, or Scoter, I could see in Liverpool Bay. For at least a couple of miles a great raft of these sea duck extended over the choppy waves. There must have been many thousands of birds there, and another raft of some ten thousand duck extended over the Formby Channel when I moved farther up the Lancashire coast. Possibly a strong westerly wind blowing at the time held up their usual September migration through the Irish Sea. On another occasion, during a very severe frost, it was vividly impressed upon me that plovers must travel far in search of the open marshes of the coast, for on the coastal plain two enormous flocks of Lapwings momentarily joined together until the sky was full of their broad, flapping, almost black and white wings. I estimated ten thousand Lapwings there, and I watched to see if their wailing cries were part of a rivalry between the two flocks over such useful feeding ground, but shortly afterwards the flocks broke up again and departed. For several successive winters I used to watch a white Oyster-Catcher in the Dee Estuary, and as white Oyster-Catchers are not, by any means, common, this was probably the same bird returning each year to the same winter haunts at Hilbre Island. Merely to watch and note the names of birds misses half the fun in bird watching. Once you have learned to know birds, a closer scrutiny will sometimes reveal one of the Continental races that can be distinguished by sight, like the larger, more highly-coloured Greenland Wheatear, the paler Continental Song Thrush, the white-headed Northern Long-tailed Tit, or Continental forms of Goldfinch, Bullfinch and Great Tit with their brighter colours. Perhaps a Greater Spotted Woodpecker whose whiteness is clear white, not the dirty white of our native bird, will be the Northern or Con¬tinental form? In the Eastern counties such -differences are most often seen, but there is always interest in looking twice at even the commonest bird. I have seen the members of a field club on one of their afternoon walks dismiss a party of White Wagtails under the illusion that they were Pied Wag¬tails ; often a flock of Yellow Wagtails on spring migration will repay closer attention by the discovery that they include one or two of the blue-headed form. We hardly ever see the shore in winter without its noisy mob of Herring-Gulls: but are we sure they are all Herring-Gulls, and that there is not one without the black tips to its wings? And then by the wing tips extending to or beyond the tail we can deter¬mine if the visitor is a Glaucous Gull or less frequently an Icelander. When great flocks of Golden Plover foregather in April before their homeward migrations, the Northern race can be determined on sight by the enormous extent of black edged with white on the cock bird's underparts. The bird-watcher will look twice at the legs of a Ringed Plover to make sure it is not a Kentish Plover, and so on. He who has learned to look at small details will notice the seasonal changes of plumage, and sex and age his birds by such means. He will see minor changes such as the Oyster-Catcher's white neck-band in winter, and the cock Fieldfare's yellower bill in May; he will notice how many of the roving party of Long-tailed Tits wear the brown tints of juvenile days. But when bird-watching is organised at suitable places at the same time, it is possible to learn how extensive is a migration, like that of Tern passing inland in May. This avoids the danger of making generalisations from casual and local records. Places of equal migration dates can then be linked on a map by lines called migrant isophenes. Thus we see how the spring migrants invade progressively the South-West, the West, and the South, and later the Midlands and the North-East of Britain, keeping close to the spread of spring weather, while the autumn arrival of the Fieldfare is almost the reverse, starting with North-East Scotland. Such migration maps can be compared very easily with the physical conditions and the weather which influence the invasion. Prehistoric migration routes may be located from prehistoric bird remains unearthed in places which may not always coincide with their present range. An ornithologist who is also an archaeologist told me that by this means, remains of the Lesser White-fronted Goose in Sweden indicated that' this scarce British visitor had changed its nesting range in modern times.

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