282 The Ruffed Grouse 



in 1941. Productivity was normal from 1937 to 1939 for increasing 

 populations. The very low productivity in the summer of 1940 pre- 

 ceded a moderate decline in 1941. 



Reviewing the trends of the whole twelve-year period, the cor- 

 relation between breeding density and subsequent recovery seems 

 to be of basic importance. They appear to fall into three groups: 

 ( 1 ) low breeding populations that have a low recovery rate; (2) low 

 breeding populations that are followed by a high recovery rate, or 

 moderately high populations having a high recovery; and ( 3 ) high 

 populations (or occasionally only moderately high ones) that are 

 followed by poor recoveiy rates (see Fig. 12). 



Generalizing these correlations, we may regroup them into two 

 series in order to illustrate the inverse ratio between breeding den- 

 sity and productivity in its two opposite phases. Combining the 

 first two groups as those having low-to-moderate breeding densities 

 ( 1931 almost falls with the second group anyway and 1930 had so 

 low a population that it was not yet able to attain a high reproduc- 

 tive rate, thus making it an exception to the principle involved), 

 we see the contrast with the other group of years having high breed- 

 ing populations that lead to low recoveries. 



Errington's records on marginal giouse populations at Prairie du 

 Sac, Wis., suggest the same three phases, but here too the details 

 are somewhat obscured by inadequate data (letter from Errington, 

 January 21, 1942). 



The inverse ratio between breeding density and productivity is 

 also shown for gallinaceous species other than ruffed grouse. Er- 

 rington's data on bobwhite quail ( a noncyclic species ) in southern 

 Wisconsin, R. E. Yeatter's work on prairie chickens and bobwhite 

 in Illinois, and that of Arthur Hawkins and others on mixed popu- 

 lations of bobwhite, Hungarian partridge, ring-necked pheasant, 

 and prairie chicken in Wisconsin, all illustrate the principle (Aldo 

 Leopold and Paul L. Errington, MS). 



From the consistent evidence we may conclude that the losses 

 of birds that succumb following seasons of high-breeding density 

 are destined to happen from some cause as a result of the population 

 condition regardless of what the actual and immediate cause of 

 death may be. They are vulnerable as a result of their own num- 

 bers and the adjustment in population level is in the cards, irrespec- 

 tive of the operation of the various mortality factors. However, one 



