September, 1945 



Brown & Yeager: Squirrels in Illinois 



453 



method gives more properly an index of 

 relative abundance in the various units 

 rather than a true census. Later, Dice 

 (1941) summarized various census tech- 

 niques, some of which were applicable 

 directly or indirectly to squirrels. Leo- 

 pold (1933) discussed census methods for 

 both game birds and mammals. Like 

 Dice, he suggested techniques that could 

 be used in squirrel census. 



Goodrum (1940) developed three cen- 

 sus techniques for gray squirrels in Texas, 

 which probably would be equally appli- 

 cable to fox squirrels. These methods 

 were as follows: 



1. Time-area counts, using number of 

 squirrels observed during definite time 

 period on random plots of known size in 

 each ecological type. 



2. Nest counts in each ecological type, 

 multiplied by a factor of two. 



3. Hunting with dog, using number of 

 squirrels treed per hour. 



The first method is based on the average 

 distance a squirrel can be seen, and thus 

 the size of the territory that may be ob- 

 served from a stationary position. Good- 

 rum, using three-fourths of this territory 

 as the unit that could be watched effec- 

 tively, calculated populations in each ec- 

 ological type by multiplying the average 

 number of squirrels seen per acre on each 

 observation territory by the number of 

 acres in the type represented. The second 

 method involved counting all twig and 

 leaf nests, preferably in the fall and win- 

 ter, in each ecological type and multiplying 

 by two. The third method, employed 

 only when squirrels were active on the 

 ground, gave the number of animals treed 

 per hour in each ecological type. In each 

 of these methods, the greater the number 

 of counts and the more uniform the vege- 

 tational types, the more nearly accurate 

 is the census. 



It should be noted that Goodrum's first 

 two methods result, theoretically at least, 

 in a true census ; the third, unless several 

 factors not given are known, could not be 

 more than an index of relative abundance. 

 The present writers made limited use of 

 the time-area method and were convinced 

 of its possibilities provided the investigator 

 had time for making a large number of 

 counts. In agreement with Allen (1942, 

 1943), who worked with fox squirrels in 

 Michigan, we are uncertain of the value of 



of the nest-count method. The hunting- 

 with-dog method was not tried. It is 

 probably significant that Goodrum refers 

 to these census techniques as " . . . methods 

 found most effective in estimating gray 

 squirrel populations in eastern Texas." 



Chapman (1937, 1938«) in Ohio, and 

 others, have used data taken directly from 

 hunters as a means of determining trends 

 in populations. This is a rapid and use- 

 ful method, and, if based on many samples, 

 may indicate trends for large areas, even 

 for a state as a whole. However, most 

 kill data are subject to too many variations 

 to yield more than very general informa- 

 tion, thus limiting materiallv their use. 



Baumgartner (1940), Allen (1942, 

 1943) and the present writers determined, 

 by exhaustive live trapping, and with a 

 relatively high degree of accuracy, squirrel 

 numbers on small study areas. This tech- 

 nique, however, is too time consuming for 

 practical, large scale censuses. Estimates 

 of the population made by less exacting 

 means, if based on experience and judg- 

 ment, serve many purposes of manage- 

 ment; and, if made uniformly year after 

 year, may indicate population trends satis- 

 factorily. Some experienced hunters de- 

 velop almost uncanny accuracy in judging 

 squirrel numbers on areas with which they 

 are familiar. 



ACKNOWLEDGMENTS 



For aid in various aspects of this in- 

 vestigation the writers are indebted to the 

 following organizations and individuals. 



Illinois Natural History Survey: 

 Dr. T. H. Frison, Chief, for general 

 supervision ; Dr. George W. Bennett, 

 Limnologist, for several photographs and 

 for aid in collecting; Dr. Carl O. Mohr, 

 Associate Entomologist, for suggestions in 

 regard to field work; Dr. H. H. Ross, 

 Systematic Entomologist, for identification 

 of external parasites; Dr. D. H. Thomp- 

 son, Zoologist, for assistance with statisti- 

 cal analysis; Mr. Frank C. Bellrose, Jr., 

 and Dr. Ralph E. Yeatter, for numerous 

 records and suggestions; and Mr. James 

 S. Ayars, Technical Editor, for several 

 photographs and for assistance in the 

 choice of illustrative material. 



Illinois Department of Conserva- 

 tion : Hon. Livingston E. Osborne, Di- 

 rector, and Mr. Anton J. Tomasek, State 



