NOCTURNAL MIGRATION 51 



of the ear. Some in fact seem barely above the 

 trees. Where small birds have been seen flying by 

 day, as at Point Pelee, Ontario, they are usually at 

 very moderate heights, and numerous recorders of 

 migrants crossing stretches of open water note them 

 frequently as passing barely above the waves. Re- 

 cords from lighthouses also indicate birds flying at 

 comparatively low levels, as do notes made on 

 diurnal movements among small birds at islands. 

 Eagle Clarke, in observations at the Kentish Knock 

 lightship, in the North Sea off the mouth of the 

 River Thames, noted small birds of various sorts 

 passing barely above the crests of the waves — a 

 sort of flight that prevailed even when there was fog 

 or mist, when it would have seemed advantageous 

 to attempt to rise to an altitude where the air was 

 clear. 



Weather conditions must affect the heights at 

 which birds travel, as on clear warm days, when 

 warmed air is rising from broad areas, — such days 

 for example as birds of powerful flight use in soaring, 

 — it would be easy to fly at high altitudes, while in 

 periods when the atmosphere is dead and lifeless, 

 high flight would be more difficult. Since rising air 

 currents ordinarily prevail only during the day, and 

 slacken or disappear by night, we may see in this an 

 indication that birds do not travel at excessive alti- 

 tudes after nightfall. 



