14 TH BE VAUD UC BeO Ne BY Us Lye Tia 
Weather Affects Fall Migrations 
By Ho.Liy REED BENNETT and JOHN HELMER 
Ieprror’s Note: The following article was prepared by John Helier on the 
basis of his own observations along the Chicago lake front and the detailed 
analysis previously published by Mr. Bennett, Secretary-Treasurer of the 
Chicago Ornithological Society, in the autumn, 1952 Wilson Bulletin. 
The effects of weather patterns on bird migrations, and particularly on 
“waves” of fall migrants, are clearly noticeable on the Chicago lake front. 
tecords of observations in Lincoln Park by Mr. Bennett, maintained over a 
period of many years, show that in the Chicago area the largest waves of 
migrating birds in autumn build up with the arrival of cold fronts, followed 
by northwest winds. 
These cold fronts are produced by high barometric pressure areas moving 
down from western Canada across the great plains, passing west and south 
of Chicago, with the line of the cold front extending in a SW to NE direc- 
tion. As the high pressure area moves past Chicago in a southeasterly direc- 
tion, the wind usually shifts back to southwest. 
Under these patterns, Lake Michigan acts as a barrier that concentrates 
the birds in large numbers. The northwest winds bring the migrants to the 
lake, where they turn south and follow the shore line, instead of scattering 
before the wind. ; 
The conditions most favorable for a big influx of a given species are: (1) 
when the cold front sweeps down during the early part of the migration 
period of that species, and (2) when the migration period falls late in Sep- 
tember or through October. As fall progresses, the weather patterns are 
more pronounced, as in winter. Early August and September migration 
waves are less pronounced because the weather is still mild and effective 
cold fronts are infrequent. 
If (as sometimes happens) the high pressure area passes north of Chica- 
go, the prevailing wind directions are north or northeast, and definite migra- 
tion waves do not occur. Weather maps, such as those published by the 
Chicago Tribune, often indicate when a wave of birds might be expected. 
While these maps are based on forecasts rather than on actual conditions, 
they are adequate for determining the prevailing wind patterns in the fall. 
847 Ridge Avenue, Evanston 
FI ft er 
Farm Donated to DuPage Forest Preserves 
Mrs. AUDRIE ALSPAUGH CHASE, 503 Hillside Avenue, Glen Ellyn, widow of 
Al Chase, a former real estate editor for the Chicago Tribune, has donated 
Willow Brook Farm to the DuPage County Forest Preserve Commission. The 
43-acre tract, on Park Boulevard about one mile south of Roosevelt Road, 
will be added to the existing forest preserve; it is said that part of the area 
is to be set aside as a bird sanctuary. Mr. Roy Blackwell, president of the 
forest preserve district, said this was the first tract donated to the system 
in 56 years. The Directors of the Illinois Audubon Society have written Mrs. 
Chase a letter of appreciation for her public spirit. 
