452 



Illinois Natural History Survey Bulletin 



Vol. 23. Art. 5 



during the 2 years, of which onlj' 236, or 

 7 per cent, were returned. The writers 

 consider that the most valuable informa- 

 tion yielded by the questionnaire pertained 

 to population trends and hunters' opinions 

 of squirrel seasons. 



Data pertaining to breeding were ob- 

 tained directly from examination of squir- 

 rels of all age classes and of both species. 

 Such information was recorded systemati- 

 cally under appropriate headings, as shown 

 in fig. 3. For males, the information 

 particularly sought was size and weight 

 of testes and size of Cowper's glands ; for 

 females, evidence of oestrus, pregnancy, 

 lactation and placental scars. The age 

 of all specimens was determined with as 

 much accuracy as possible, and, throughout 

 the study, special efforts were made to 

 develop aging criteria. Weights and 

 general measurements were taken of nearly 

 all specimens. Other records related to 

 locality and date of collection, condition 

 of pelage, parasites, disease and stomach 

 contents. Supplementing these data were 

 detailed observations on foods and feeding 



habits, activity, behavior, habitat require- 

 ments and other matters of ecological in- 

 terest. 



Unfortunately, no fully satisfactory 

 technique has been developed for censusing 

 squirrels. That such a tool is important 

 is obvious because the first information 

 needed in management relates to the 

 number of animals present on each study 

 unit. This information is most useful 

 when it supplies numbers at a definite 

 time; for example, population in the fall. 

 In the absence of standardized methods 

 for determining squirrel numbers, it be- 

 comes necessary to make the best estimates 

 possible. These estimates are generally 

 made by rules-of-thumb, or by using one 

 or more of the various techniques that 

 have been described. The result is often 

 expressed in reference to the population 

 by such terms as "plentiful," "common," 

 "scarce," or "heavy," "medium," "light." 



Dice (1931) proposed, as the best 

 method of determining mammal abun- 

 dance, the listing of individuals observed in 

 each ecological unit of the habitat. This 



