CAUSES OF ANIMAL CYCLES 197 



have already been given in connection with the discussion of biotic 

 balance (page 172). Leopold has brought together the data with 

 respect to the fluctuations in ruffed grouse and prairie chicken, which 

 appear to be in general agreement with the maxima and minima of 

 the sunspot cycle. In summing up the situation as to grouse, Leopold 

 and Ball conclude that American and British grouse fluctuate rhythmi- 

 cally, but the period is different. In America it averages 10 years 

 with a range of 9-14; in Britain the average is 6.5 and the range 4—8 

 years. The differences between the two countries are thought to refute 

 the assumption that cycles are due to variations in solar radiation or 

 in sunspots, but this does not reckon with the evidence that sunspot 

 and climatic cycles are regularly or frequently opposite in different 

 parts of the world, as well as with the little-understood half-cycle, 

 which appears or disappears from time to time in the same region. 



]\Iiddleton (1934) has examined the records of shooting on several 

 English estates with respect to cycles and finds an approximate cycle 

 of 8 years in the partridge (Perdix) and of 6 years in the grouse 

 (Lagopus). Woodcock (Scolopax) and blackgame (Tetrao) were also 

 found to exhibit distinct fluctuations of a cyclic character. 



Wing (1935) has made use of the records of brant shooting com- 

 piled by Phillips (1932), averaging the annual "bags" for four periods 

 to obtain the following figures: 1867-76, 405 birds; 1877-88, 169; 

 1889-98, 276; 1899-1909, 231. He concludes that the largest number 

 of brant were killed during the highs of the Briickner cycle and that 

 this coincides with the maximum population. However, as Phillips 

 points out, other factors than relative abundance enter into the size 

 of the annual bag, and there is considerable question as to whether 

 the data are sufficiently accurate to yield dependable results. This 

 is further indicated by the statement that the number of brant ob- 

 served was never greater than in 1887, though the bag was but 380 

 by contrast with 9 other years when it ranged from 410 to 715. Wing 

 has further compared the curve of numbers for prairie chicken, ruffed 

 and sharptail grouse, and house wren with that of the sunspot cycle 

 and states that the highs occur at or near the year of sunspot maxima 

 and minima. 



Cycles in Insect Populations. The locust plague so far transcends 

 all other fluctuations in insect numbers as to warrant almost exclu- 

 sive consideration of it, especially when the drama of migration is 

 taken fully into account. Though at present determination of num- 

 bers is a regular practice, in the past the phenomenon of migration 

 and its effects constitute the best and often the sole mark of the maxi- 

 mum of population. In spite of the distinct progress made during the 



