24 Jan 2010

HOW BIRDS MIGRATE

This is an interesting article on bird migration through observations made in the the fifties by
Eric Hardy a very respected Ornithologist.
How high do birds fly when migrating ? how far ? and how fast? These questions spring quickly to the mind of the lay-man newly interested in the subject, and they have engaged the attention of ornithologists for many years. The height depends upon the visibility, the speed upon whether or not the birds are aided by a following wind, like the Trans-atlantic Lapwings, while the distance (and speed) are often a question of how far north is the nesting place. Most migration, however, is below 3,000 feet, although I sat on the summit of Mt. Hermon one afternoon at the end of July, and watched and listened to flocks of European Bee-Eaters and Swallows passing overhead, at least 10,000 feet high. The bubbling songs of migrating Bee-Eaters usually announced their presence, on spring or autumn migration, high in the sky of bugloss blue above my office in Jerusalem, when they were travelling more than 3,000 feet high. They were, I believe, the "singing birds" referred to by Solomon in his remarks upon the season of spring migration. But the Bee-Eaters and Swallows dipped down much lower into the Jordan Valley after they had cleared Mt. Hermon. Lapwings have been recorded at 6,500 feet, Golden Plover at 6,000 feet, and grey geese at 5,000 feet; but on misty nights the migration must be very low, considering the frequency with which we hear their calls. Birds usually fly at a slightly faster speed than normal when migrating. For example the Swallow averages 23 to 38 m.p.h. on normal flight, and 34 to 40 on migration; and the Rook on migration increases its average from 24 to 3 5 to 3 8 to 45. Radar-timed Mallard, Pintail and Blue-winged Teal averaged 30 to 35 m.p.h., and grey geese Radar-timed nearly 80 miles averaged 25 m.p.h. In calm weather most small birds average about 25 m.p.h. Autumn migration is often very leisurely. I have known a pair of Green Sandpipers, on autumn passage-migration, remain a fortnight at the local sewage farm before continuing, and in the mild winter of 1948-1949 two remained all winter by some ponds in south Lancashire; likewise with Ruffs, Bee-Eaters and other un-common visitors. A Turnstone was shot at Cherbourg twenty-five hours after it was ringed at Heligoland; a Swift took seventeen hours at the most to travel between Algiers and England. A wandering Albatross was killed twelve days after being ringed 3,150 miles away. By collecting the records of numerous observers on the Continent, Southern has shown that the Swallow took about 109 days to cover the 2,500 miles from Gibraltar to Norway. Its progress was slowest at the beginning of the journey, when it took about a month to reach France alone, and then its numbers spread north faster at about 25 miles per day over Europe. The Willow-Warbler covered the same distance in about eighty eight days, also slowly at the beginning, when the spring weather was slow in arriving, and averaging 29 miles per day. The Redstart migrated the same distance in about sixty-one days, but this was later in the season when the weather was not of a nature to hold it up so much as had delayed the others. It averaged 30 to 40 miles per day in advancing. These speeds may seem to contradict the previous paragraph, but the former is direct flight, the latter is penetration northwards. Much of that travelling was no doubt day migration. Night migration seems most consistent where the birds require the daytime for feeding. The movement of wild duck, Woodcock and other birds is much influenced by moonlight. It is often possible to witness day migration with the Starling, Rook, the Jay (south-eastern England), and the auks. In Palestine where the Starling is only a winter visitor, I once witnessed the day migration of this bird down the deep Jordan Valley, when motoring from Jerusalem to Jericho with Dr. T. A. Cockburn, assistant superintendent of London Zoo. We had just begun to drop below Bethany in the afternoon of 21st November 1945, when in the distance over Jericho appeared a whirring flock of Starlings like a haze of locusts, flying south. An observer ringing birds at a lighthouse on the French coast near Cherbourg at night in September found the Kingfisher amongst the migrants which had almost certainly crossed the Channel in the dark. I have watched for migrants from the cross channel steamers and the birds seem to cross open sea out of sight of land-marks on a fairly broad front. But some seabirds like Shearwaters appear to use "lanes" at sea. Few birds are more conspicuous in the sky than the Swift which the late Edward Thomas described:
With wings and tail as sharp and narrow as if the bow had flown off with the arrow.
Its sudden migration, so quickly noticed, often coincides with a rise in temperature at the full moon in August.

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