2C2 ' NATURAL SCIENCE. April, 



meteorological conditions prevailing during the movements in most 

 instances contributes in no small degree to a correct interpretation of 

 their precise nature and the seat of their origin." Unless, therefore, 

 the conditions at the point of departure are favourable to emigration, 

 or are such as to render the journey imperative, it is either deferred 

 or undertaken most reluctantly. 



These weather conditions are of an exceedingly complex nature, 

 and Mr. Clarke is at some trouble to explain their origin and effects 

 upon migration : under ordinary conditions the migratory movements 

 proceed as usual ; in somewhat unsettled weather they may be in some 

 degree quickened, but are still normal ; exceptionally stormy weather 

 arrests all migration, and the barred-up stream is detained for a 

 favourable opportunity ; its eventual release results in one of those 

 "rushes," with which dwellers on the east coast are so familiar, but 

 which never fails to excite their astonishment. 



Temperature is also an important factor in these movements, and 

 indeed a decided fall of the thermometer is a frequent accompaniment 

 of the anti-cyclonic conditions with atmospheric calms, so highly 

 favourable for sustained flight. Of course these conditions apply 

 equally to both spring and autumn movements. It does not follow, 

 however, that the favourable weather prevailing when the journey is 

 commenced will be sustained during the whole of its course, and 

 possibly the migrants may encounter very unsuitable weather before 

 reaching their destination ; under these circumstances they arrive in a 

 state of great exhaustion, and many individuals probably perish 

 en route. 



Fog, which often prevails during the autumn and winter periods 

 of flight, appears greatly to inconvenience the migrating birds, and it 

 has been noticed that even non-migratory birds, such as the sparrow, 

 have occasionally lost their bearings through it. 



The simultaneous autumnal influx and efflux already mentioned 

 has been found to occur in periods when anti-cyclonic conditions 

 prevail over an area sufficiently wide to cover both the points of 

 departure and arrival, the result being "a great simultaneous 

 inpouring of birds on our east coast, and a general outpouring from 

 all British coasts of migrants of many species." The comparatively 

 simple east to west route and the more complex northerly route are 

 both governed by the same meteorological influences, and take place 

 chiefly during calm and cold anti-cyclonic periods, being interrupted 

 by rough weather and renewed with increased energy on the return of 

 suitable atmospheric conditions. 



Another result of the study of the returns is the more just 

 appreciation of the influence of the winds on migration. Mr. Clarke, 

 after a careful study of the evidence, arrives at the conclusion " (i) that 

 the direction of the wind has no influence whatever ^5 an incentive to 

 migration ; but that (2) its force is certainl}^ an important factor, 

 inasmuch as it may make migration an impossibility, arrest to a 



