The Autumn Flight of BirdsAs the year swings round towards the autumn equinox,our migrant birds, the molt over, begin to evince a new restlessness. The climatic conditions of their environment are now in a state of change, which is not unlike that of early spring except in one important particular ; the days are now getting rapidly shorter, instead of longer. The same cold nip is apparent in the early morning air; there is a tendency for wide alternations of heat and cold, and just as the greatest variations in temperature in spring (which occur in May and early June) correspond with maximum sexual activity among birds, so it seems possible that the wide fluctuations of autumn may affect the birds in an analogous, but reciprocal, manner, and induce the tendency to pack together in flocks, or to depart once more on the long journey back to the tropics. Before the main tide of summer migrants sweeps down and out of this country, there are several phenomena of a secondary character which are worth noticing. One is the phenomenon of "dispersal" which shows itself by the scattering of the birds of the year over the countryside. Some of these young birds may make journeys which are most puzzling, and go northwards, instead of south, in the first instance. Thus a young sandwich tern, ringed in Fife on June 29th, was found in Aberdeenshire, 65 miles to the north on July 1st. Before the end of July, a bird ringed in Cumberland had reached Perthshire, 155 miles northward, whilst a Norfolk bird reached Angus 285miles northward. Other young sandwich terns went south,however, and the scatter, although it has a slight northerly bias, appears to be fairly general around the points of the compass. Another phenomenon of the pre-migration time of early summer is the tendency of certain birds to begin to pack into flocks for roosting at night. The wagtails show this habit to a surprising extent, and pied wagtails will even begin to roost together in large numbers as early as midsummer. Yellow wagtails will commence roosting in flocks in August and September prior to departure. These communal roosting flocks appear to be composed of both young of the year and adult birds, and we know comparatively little about the reason for, or significance of, this habit.
One of the most baffling problems of the flight of the autumn migrants is the unaccountably early departure of the swifts. No sooner have these birds completed their breeding cycle than the restless urge to be off and away becomes increasingly evident. At the end of July, they pack together in great black flocks and sweep across the sky at evening, screaming as they go. In a seeming frenzy of effort they dash round their church steeples, or mount in ever-heightening circles into the sky. Then, on an early August morning, following a night when, perhaps, the wind has swung to the north and the temperature abruptly dropped, we find but few left in the skies around us. Why the swifts should leave thus early, when as far as we can judge, insect life is plentiful, and likely to remain so, is an unsolved mystery. The swallows and the two martins will be with us a further two months, and may indeed, still have young in the nest. Is it a question of a highly specific food supply which fails at this period of the year, or is another and as yet unknown, psychological factor at work? Again, we do not know. The answer may possibly be arrived at one day; in the meantime observations on the mode of departure of the swift and the conditions attending it are of great value. One observer recently carried out a census of the number of swifts to be seen in the sky over the town of Oxford during the evenings of the second week in August, by mounting to the top of a high building and taking a " snap " count of the greatest number that could be counted at one time on any one night. The numbers from August 8th to 11th were fairly constant at 11 to 14 birds. During this time the weather was fine and the wind westerly in direction. On the night of the nth to I2ih August, it changed to the north, and the sky became overcast for a while but was then clear again with the wind north-easterly, and on the 12th the number of swifts counted by this method dropped to 5. More on the Autumn Flight of Birds..... |
