and very slowly. It is probable that strong air-currents at a high elevation materially assist rapid and lengthened migration. Force not direction of wind influences birds moving at a low elevation.
Birds possess a certain power of orientation, a homing instinct, which need not be called a sixth sense. Brain and eyes assist in the development of this power; birds have an excellent memory. Young birds lose their way more frequently than is generally supposed; variations in routes are explained in many cases by these errors. Young may or may not be guided by experienced adults; orientation is not infallible but develops with age.
There is apparently no truth in the assertion that birds travel by choice against a head wind or in a beam wind; a moderate wind behind, on which they are carried, is most favourable. Leeward drift through contrary winds explains many normal and abnormal routers, and the occurrence of unexpected species in unexpected places. The distance travelled not only varies according to species but in individuals of the some species; the thesis that the most northerly breeder winters furthest south does not always hold good.
Much may be learnt by the careful registration of arrivals and departures of migratory birds, and by the marking of birds. Ornithophænology, the science of migration study, as carried on at present
