. Studies in bird migration . tishIsles A large Continental anticyclone lies to the east of the British Isles, andextends to our shores. To the west the pressure decreases, especially offthe Atlantic Coast of Ireland and over Iceland. Wind, a south-easterlycurrent over the United Kingdom. The south-easterly type of weather is the most favourable of all for inter-migration between Great Britain and the Continent of Europe in both springand autumn. During its prevalence the great migratory movements, termedrushes, are performed. Fine weather is then in the ascendant over the countries whence the
. Studies in bird migration . tishIsles A large Continental anticyclone lies to the east of the British Isles, andextends to our shores. To the west the pressure decreases, especially offthe Atlantic Coast of Ireland and over Iceland. Wind, a south-easterlycurrent over the United Kingdom. The south-easterly type of weather is the most favourable of all for inter-migration between Great Britain and the Continent of Europe in both springand autumn. During its prevalence the great migratory movements, termedrushes, are performed. Fine weather is then in the ascendant over the countries whence thegreat majority of our bird-visitors come to us in the autumn, and to whichthey return on leaving us in the spring. The North Sea is spanned, at bothseasons, by weather which is most promising for the voyagers. On the other hand, the western low-pressure area, with its unsettledconditions, extends to Iceland, and is against intermigration between theBritish Isles and that great north-western island. (See also p. 173.) PLATE Reduced from Special Chart prepared at the Meteorological Office.] South-Easterly Type of Weather. Isobars are shown by black lines, with indications of the height of the —Arrows flying with the wind, show direction and force, thus :— ^ = forces 5 to 7 (velocity, 28 to 40miles per hour). iw -- forces I to 4 (velocity, 8 to 23 miles per hour). Vol. I.] [To face p. 172. WEATHER INFLUENCES 173 migrants or would-be migrants. It must first be re-marked, however, that the direction of the wind hasin itself nothing to do with the results described. Thewinds and the performance, or non-performance, of themigratory movements are the effects of a commoncause—namely, the particular type of weather prevailingat the time, which may be favourable or unfavourablefor the (light of birds to or from our islands. Favourable Weather Conditions. — The favourableconditions prevail when a large anticyclone has itscentre in North - Western Europe, covers t
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