The whole problem of Migration is still a mystery, but an enhancement of this mystery is in the irregularity and incompleteness of the working out of this all but universal instinct, this inscrutable rhythmic law. Both the skylark and the blackbird, for example, are migratory birds, and yet larks and merles by the thousand remain in our northlands through the winter, and even come to us at that season. The skylark in particular puzzles the ornithologist. While certain birds appear and disappear with an astonishing regularity, as though they heard the pealing of aerial chimes afar off and knew the bells of home ... the swallow, for example; or, again, the tiny gold-crested wren, in some parts called ‘the woodcock-pilot’ because in two or at most three days after its appearance the first woodcocks are invariably seen ... there are others, like the song-thrush, which will pass away in the great migratory clouds that like withdrawing veils every autumn carry the winged clans oversea; which will pass so absolutely that for a hundred miles not one of its kind will be observed, not even a straggler: and yet, in some other direction, others will be seen weeks later and perhaps even through the winter. We are all familiar with the homestay of the Redbreast, and many people believe that it is not a migrant because of its frequency about our garden-ways even in the hardest winter: and yet, in incalculable myriads, the redbreast migrates as far south as the Sahara, and its sweet home-song of the north may be heard in Greece, by the banks of the Nile, throughout Palestine even, from the cedars of Lebanon to the valleys about Jerusalem.