56 MANNER OF MIGRATION. highways of migration. Tlirough telescopic observations it has been learned that migrating birds travel at a great height. The exact height remains to be determined, but it is known that many migrants are at least a mile above the earth. From this elevation they command an ex- tended view, and in clear weather prominent features of the landscape are doubtless distinguishable to their pow- erful vision at a great distance. It is when fogs and storms obscure the view that birds lose their way. Then they fly nmch lower, perhaps seek- ing some landmark, and, should a lighthouse lie in their path, they are often attracted to it in countless numbers. Thousands of l)irds perish annually by striking these lights during stormy fall weather. In the spring the weather is more settled and fewer birds are killed. Although birds are guided mainly by sight, hearing is also of assistance to them on their migrations. Indeed, at night, young birds, who have never made the journey be- fore, must rely largely upon this sense to direct them. It is difficult for us to realize that on favorable nights during the migratory season myriads of birds are passing through the dark and apparently deserted air above us. Often they are so numerous as to form a continuous stream, and if we listen we may hear their voices as they call to one another while flying rapidly onward. Some idea may be formed of the multitude of birds which throng the upper air on favorable nights during their migration by using a telescope. One having a two- inch object glass will answer the purpose. It should be focused on the moon, when the birds in passing are sil- houetted against the glowhig background. At the proper focal distance they appear with startling distinctness. In some cases each wing-beat can be detected, and with a large glass it is even possible to occasionally recognize the kind of bird.
