MIGRATIONS OF WATERFOWL 



and the barometer was on the rise. Thus, on the afternoon of October 30, 

 1954, the heaviest passage of the year crossed Delta during a period favor- 

 able in every respect except for repeated snow squalls. But despite the rising 

 barometer, the northwest wind, and the steady stream of migrants passing 

 over during most of the afternoon and early evening, none of the local ducks, 

 concentrated in many thousands, were seen to leave. Next day the weather 

 cleared and by evening the sky was bright. The local ducks started to mi- 

 grate at about 4:00 p.m., and by sundown we had seen a tremendous move- 

 ment away from Delta. The waterfowl that passed over in storm on October 

 30 no doubt had started from their more northern marshes under the clear 

 sky of the high-pressure system that did not bring fair weather to Delta until 

 the next day. I take it from this and from many similar observations that 

 open sky as well as favorable wind is important at the start of mass migra- 

 tions. 



But then, if clear skies are a prerequisite for the beginning of mass mi- 

 gration, the passages cany on, nevertheless, to overtake and fly through bad 

 weather. The speed of migration is the product of the birds' air speed plus 

 the velocity of the wind ; hence the migrants travel faster than the weather 

 itself, often, as in the passage of October 30, 1954, catching up with the 

 region of storm ahead. Many times I have observed ducks arriving at Delta 

 in bad weather that was followed shortly by the broad edge of cloud intro- 

 ducing clear skies and fine anticylconic weather. They often passed right on 



