ECOLOGICAL FACTORS 



397 



are deflected upwards ; there one regularly sees the gulls soaring with- 

 out beating their wings. Areas where low barometric pressure is almost 

 stationary, as for example, along the east coast of the Adriatic or over 

 the Tyrrhenian Sea, are characterized by ascending air currents. 

 Ascending currents also originate by solar irradiation in places where 

 areas with different heat capacity lie in close approximation; thus 

 forests warm up or cool off more slowly than grasslands or fields ; water 



e=sc? 



Fig. 114. — The three principal migration routes of European birds; the 



west coast route, the Italo-Spanish route, and the Adriatic- 

 Tunisian route. After Lucanus. 



and land have a similar relation. As a consequence the air rises over 

 the grassland or shore by day and over the forest or water by night. 

 Such air currents are present especially on seacoasts and in river val- 

 leys. It is not improbable that for this very reason such stretches are 

 used as the migratory routes of birds, since they present narrow chan- 

 nels in the ocean of air. 09 Large groups of birds of various species 

 travel these routes independently (Fig. 114) ; in the Old World such 

 routes lie along the coasts of Scandinavia, of the Baltic and North 

 seas, of Spain, Italy, the Balkan Peninsula, and Syria, or they follow 

 the Rhine, Vistula, Danube, or Nile; in North America, the Mississippi 

 and both coasts are important migration lanes. The problem of migra- 

 tory routes thus becomes, in part, a meteorological one. 



