546 PRODUCT n IT) OF CROVS,E POPULATIONS 



begun in 1932 and for lliree years the breeding populalion experienced net losses. From 

 1935 to 1936 it just maintained itself and then gaiiiod jiliphtly during the next two years. 

 After 1938 spring surveys were discontinued but data from summer work each season indi- 

 cate that a still greater gain occurred in 1939 followed by a minor decline to a level which ap- 

 parently varied little in 1941 and 1942. Of particular interest are the years 1934-35 and 

 1935-36. As a result of above average survival throughout the breeding, nesting and rearing 

 seasons the fall population of 1934 was the highest recorded. But during the ensuing winter 

 the availability of buffers reached its lowest ebb and winter mortality rose to 71.2 per cent. 

 Following this, excessive losses among the chicks in 1935 had reduced productivity to almost 

 zero by September. Yet with buffers still low winter mortality dropped to 8.7 per cent. 



These observations lead to the conclusion that, in habitats of the Northeast in which sub- 

 stantial changes in the character of the environment do not occur, breeding populations of this 

 species tend in the long run to fluctuate about a level of abundance determined chiefly by the 

 quality of the coverts involved. Net productivity from year to year oscillates above and be- 

 low this level. Manipulation by manat^ement of certain environmental elements, especially 

 shelter composition and arrangement, may raise the carrying capacity of the range. But, once 

 the birds have reached the improved plane, the tendency toward equilibrium may be ex- 

 pected to assert itself again. It must be pointed out, however, that the Investigation has had no 

 opportunity to study conditions during a period of decline and scarcity. 



Life Equations 



It is obvious that no population can continue to increase for very long without becoming 

 over-abundant, nor can it continue to decrease without disappearing. Thus the game man- 

 ager must strive for a balance between the two as well as to maintain a level of abundance 

 as close as possible to the carrying capacity of the range. Further, he must so manipulate the 

 environment that the greatest possible proportion of the inevitable losses each year result 

 from hunting for sport rather than from other causes. 



The net productivity of any population may be expressed by the equation: 



BP + MY - AL = B'P 

 in which BP = initial breeding population, MY = maturing young, AL = adult losses and 

 B'P = breeding population the following year. In a stable situation the annual increment 

 equals the adult losses. When the number of birds increases the former exceeds the other and 

 when it decreases the reverse is true. 



As an aid in roughly appraising what may be taking place on an area an attempt has been 

 made to summarize graphically the data illustrative of increasing, stable and decreasing pro- 

 ductivity (figures 52 to .56). These diagrams, however, do not re|)resenl exact records for spe- 

 cific years. Rather they constitute composite pictures of basic patterns which have been ob- 

 served to be associated with the rcs|)ective results*. Obviously iiniumcralile other combina- 

 tions of circumstances could have the same effect. But in the experience of the Investigation 

 these have occurred most frequently. 



In each case the initial breeding population (174) is that which, under an even sex ratio 

 and at the average rate of 11.5 eggs per female, would produce 1,000 eggs if breeding were 

 complete. The progressive reduction of this potential increment is then shown in the left- 

 hand circle while the other deals with tiie fate of the breeders themselves. Together the sur- 

 vivors comprise the population the following spring. 



* Althuunh hunting Ma* nut a (actor on tin* main tluily areas ita cUcrt lias bcni incuriiuratcd in llicao iJiaerams uti the basia nl 

 aupplcmrntal data (Sep Cliaptcr IX, p. ,t7B), 



