VISIBLE MIGRATION WITHIN FENNO‐SCANDIA*

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Abstract
SUMMARY.: The adáaptation of migration patterns to the climate of Fenno‐Scandia is outlined. It is suggested that food supply is the ultimate factor for all the manifold types of migration. Falling temperature is the proximate factor, releasing emigration in autumn or summer, and it causes Rückzug (reversed migration) in spring. A rise in temperature releases the spring movement or stimulates further fight, if the birds have reached temperate regions. The annual and diurnal migration rhythms of some migrants are described. Most early migrants have a two‐peak rhythm, which gradually changes to a single diurnal one in autumn. The second day‐time peak in summer is probably transformed to a nocturnal one in autumn in some species. Topography has a strong modifying iduence on the routes of diurnal migrants but much less—or none at all—on the fight of nocturnal travellers. Wind is the disastrous factor in migration. Its detrimental effect is minimized by the tendency to follow topographical lines and head the wind, at least when it is strong.