1 9 1 8 . ] Birds of the Isle of May . 25 1 



whicli rashes of birds oome to the ishmd, and uncommon 

 visitors appear, is during easterly or south-easterly winds, 

 usually with cloud, haze, fog, or rain. An extensive anti- 

 cyclonic area, with light east to south wind and fair weather, 

 brings numbers of birds, and is by (ar tiie most satisfactory 

 from our point of view, because working is agreeable and 

 one can be sure of determining a much larger proportion 

 of the arrivals than is the case in bad weather. A falling 

 barometer, with strong east winds and heavy rain, does not, 

 however, stop the birds ; we have seen large numbers arrive 

 under such conditions, when wind and rain made working 

 very difficult. In northerly and westerly winds compara- 

 tively few birds are seen, and these are almost entirely our 

 own summer or winter visitors ; the enormous rushes of 

 passage migrants do not occur and uncon)mon visitors are 

 few and far between. At the same time, we are not sure that 

 this is not the weather most favourable for the birds, although 

 not for the observer. The direction of the wind does not 

 prevent birds migrating — they move when the wind is at 

 every point of the compass, — but although this is so, in our 

 opinion the direction of the wind has a great influence on 

 the route followed and therefore on the sjjccies which occur 

 on our shores. From evidence gathered on the Isle of May 

 and from data which we have studied, extending over a 

 long period of years, in this country and elsewhere, we are 

 convinced that alternative routes are followed, according to 

 wind influences, though it is difficult to say whether drift is 

 wholly responsible for the deviation from the direct route 

 or whether this is to some extent undertaken voluntarily. 

 For instance, in the case of birds going from their summer 

 home in northern Europe to their winter quarters in northern 

 Africa, if an easterly type of weather prevail during their 

 migration period^ we find that they strike our east coasts in 

 enormous numbers, and many species are seen which do not 

 visit us imder any other weather conditions. On the other 

 hand, should the prevailing type be westerly, these migrants 

 do not strike the British Isles at all, and we believe that 

 they proceed to their winter quarters along a more easterly 



