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AERIAL PHOTO USE IN FIELDS OF WILDLIFE AND RECREATION 
superimposing on them sheets of acetate or other clear material with grills or 
blocks of the size desired marked off in fine lines. After a "block" has been ex- 
amined for ducks it can be marked with a grease pencil so it will not be counted 
twice. 
In estimating waterfowl or other birds in flight, ornithologists customarily 
count the birds in a small segment of the flock, then estimate the number of 
times this segment will go into the entire flock to determine the total. In making 
counts from a plane, however, particularly when one or more large flocks are in 
the air at the same time and frightened by the plane, this method has proven 
unsatisfactory. Spinner (1946) felt that if a means of rapidly estimating the num- 
  
Fic. 3. Large concentrations of animals such as waterfowl on their wintering grounds, spawn- 
ing salmon, or caribou in migration can be censused by means of aerial photographs. This photo 
shows Stone's caribou on the Alaska Peninsula.— Edward F. Chatelain, Fish and Wildlife Service. 
bers in small flocks could be combined with one for obtaining more accurate 
estimates of large flocks, the resulting observations would have greater value. 
Enlargements of aerial photographs showing flocks of geese of known size in 
basic flying formations, i.e., in groups and long strings, were copied on 4 by 
5 inch prints. For field use these were attached to stiff 4 by 7 inch cardboards. 
Then flocks observed in the field were compared with the photographs to deter- 
mine their approximate size. In some cases two or more photographs were used 
as accurately counted segments in arriving at the size of larger flocks which 
otherwise could not have been estimated so well. 
For use as base maps in plotting dens, cover and food in Iowa wildlife studies, 
Trump and Hendrickson (1949) have suggested dimly printed, rather diffused 
negative images instead of aerial photographs having considerable contrast and 
sharpness. They found that underexposed but fully developed “negative” 
prints revealed the landmarks essential for mapping but were dim enough to 
permit annotation and addition of details in pencil or ink. They used a balop- 
tican to project the image of a regular black-and-white print on a sheet of photo-