Bird Migration.

Kaipara and Waitemata Echo, 30 October 1914, Page 8

Bird Migration.

TWO GREAT PUZZLES

In connection with the phenomenon of the migration of birds, the two great pxi/.zlcs are—Why does it exist ? and How is it performed V The first question is not answered by saying it is a habit into which some birds have fallen for the sake of securing to themselves those means or conditions of life that are implied by "fitting temperature and sufficiency of food." Trere are birds that migrate when, to all human appearance, there is no need for doing it. The fieldfare— to take a familiar example—might as well live and breed in Scotland during the summer, so far as temperature and food supply go, as does its congener the thrush. The answer to the question must show the reason for the formation of the habit—must say why migration first began, and why it continued till the habit became an inherited instinct. It must show why certain air routes are followed that do not seem to be the best available; and why the migration is limited by barriers that are to 115 invisible. Why, for example, does the nightingale not rest north of the Tyne or west of the Severn ? The answer may be—must be in some cases —dependent on considerations 01 land-contour and climate in remote geological ages. The :ther question is not so difficult to attempt to answer. Small birds, marked in Scotland, have been found after migration as far south of the Equator as the Orange Free State ! And the goldcrest, the smallest bird that favours our country with its tiny presence, crosses and re crosses the North Sea regularly with apparent ease.

We speak with astonishment of such long flights—as we call them ; and our astonishment is increased to observe —as we sometimes do—little or no sign of exhaustion upon birds just arrived on our shores at the time of the spring migration. But the biras' journey is less a flight— i.e., an active use of the wings in flying—than a case of .'loating or drifting on a moving stream of air, going so regularly in the upper regions of the sky, at an elevation of, say, from one to three miles, as to warrant one in designating it a trade-current. Such an upper aerial trade-current—now northerly, now southerly, according to the sun's position in its enormous orbitmay move at the astonishing rate of 80, 100, or even L:"0 miles an hour. The greatest difficulty experienced by migrating birds is their ascent to this medium of conveyance ; descent being a thing of less difficulty, unless in the ower air strata there be contrary winds, against which for a landing on terra firma they may have to contend. Thousands of exhausted birds may sometimes be picked up on the Norfolk coast in the spring months, their exhaustion being caused not by the distance they have travelled, but by the adverse winds they were obliged to encounter as they descended from their aerial vectors to the summer breeding haunts, the great objective and goal of their long journey.