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l8 HOW TO STUDY BIRDS general shape of the isochronal lines. The isotherms have, in March and early April, the same tilt southwards from west to east, as have the isochronal lines of the birds, and they show the same rapid bound forward over continental Europe during April. There is thus a distinct tendency for the willow-warbler migration to keep pace with the advance of the 480 F. isotherm over Europe, although towards the end there is a tendency for the birds to outstrip the isotherm as they near their northern breeding areas. The adherence of the rate of spread northwards of birds in spring to a given temperature, is probably associated with the appearance of specific types of insect food (flies, grubs, spiders, etc.) with the advent of the warmer weather. This is shown by a consideration of the rate of spread of the swallow over Europe. The swallow arrives at Gibraltar on February 13th and reaches Varanger, in North Norway, on June 2nd. It thus takes 109 days to cover the 2000 miles, a daily rate of progress of 23 miles. With this bird there is, however, a certain lag over the Spanish peninsula, and the South of France is reached about March 15th. The rate is then stepped up slightly to average 25 miles a day up to North Norway. The delay over the Iberian sierras is probably associated with a lack of the specialised winged prey on which the swallows feed. Ornithologists in America, studying the spread of spring migrants over their continent, have shown that the ruby throated humming bird, which is a nectar eater, advances northwards at almost exactly the same rate as the 35° F. isotherm, since the appearance of this average temperature will open the flowers of certain plants on which the humming bird feeds. |
