. Bulletin. Natural history; Natural history. 394 Illinois Natural History Survey Bulletin I'ol. 22. Art. 4 population trends than the booniiny; ground censuses taken in April. For ex- ample, the study area proved to be a fa- vorite wintering: urouiui for prairie chick- ens, and each year, in earh winter, an influx of birds occurred from nearby farms, causing the population level there to rise above that of the surrounding range. In autumn the relatively high mo- bility of chicken flocks caused some varia- tion from day to day in the number of birds present on a given area of 1936 1
. Bulletin. Natural history; Natural history. 394 Illinois Natural History Survey Bulletin I'ol. 22. Art. 4 population trends than the booniiny; ground censuses taken in April. For ex- ample, the study area proved to be a fa- vorite wintering: urouiui for prairie chick- ens, and each year, in earh winter, an influx of birds occurred from nearby farms, causing the population level there to rise above that of the surrounding range. In autumn the relatively high mo- bility of chicken flocks caused some varia- tion from day to day in the number of birds present on a given area of 1936 1937 1936 1939 1940 1941 1942 Fig. 12.âTreiKl of male prairie chicken populations on a 2-by-2-mile area in Jasper County, 1936-1942, shown by booming ground censuses. although the figures obtained during the fall censuses were undoubtedly more near- ly representative of normal populations than those from the winter censuses. The fall censuses, taken annually from 1935 through 1941, usually in late October or early November, showed an average popu- lation during the 7-year period of 179 birds present on the area. The largest number of birds shown by any fall census was 255, in 1938, and the smallest num- ber was 141, in 1939. These figures represent a variation in fall population densities of from about 1 bird per 10 acres to 1 bird per 18 acres. The average was approximately 1 bird per acres. The results of the censuses of male birds on tlie booming grounds of the Jas- per County study area are shown in an accompanying graph, fig. 12. Since the number of males on any par- ticular booming ground is never constant throughout the season, every effort was made to take the census at the height of the booming season, usually in late April. Hamerstrom's (1941) work clearl> shows the need for care in this respect. The booming census is subject to criticism in that it does not include the female birds; our eff(jrts to determine sex ratif)s by trap- ping in late winter were unsuc
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