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480 



Illinois Natural History Survey Bulletin 



Vol 23. Art. 5 



Table 16. — Chronology of breeding in female squirrels in Illinois; data obtained mainly 

 July 1, 1940, to June 30, 1942. Peak dates are applicable particularly to central zone. 



First Season 



Second Season 



*Peak dates determined by averaging data from first and second breeding seasons in the first 2 years of study. 

 Pregnancy peaks are given for a 2-week, and lactation peaks for a 4-week, period. 



record for May 27 in fox squirrels, and 

 for May 31 in gray squirrels. These 

 dates marked the latest that first season 

 lactation was observed in the tw^o species, 

 and came after the earliest observed dates 

 of second-season oestrus, which were April 

 20 in fox squirrels and May 5 in gray 

 squirrels. Since it is probable that neither 

 the latest nursing nor the earliest oestrus 



was observed, overlap in the two seasons 

 may be greater than indicated in table 15. 

 This table indicates that completion of 

 summer mating in July was distinct; 

 pregnancy extended into late August in 

 fox squirrels and into September in gray 

 squirrels; lactation continued into Sep- 

 tember in fox squirrels and into October 

 in gray squirrels. 



Table 17. — Dates of first and last reproductive activity and duration of squirrel breeding 

 seasons (all zones) in Illinois; data obtained mainly July 1, 1940, to June 30, 1942. 3 



