522 PRODUCTIVITY OF CROVSE POPULATIONS 



Rate of Spread 



Yet exceeding the saturation point is not the only cause of birds shifting from one covert 

 to another. It is a basic axiom that "nature abhors a vacuum." Thus grouse spread from 

 well-populated to unoccupied or poorly stocked territory unless for some reason the latter is 

 definitely unsuitable. Because even adjacent coverts seldom follow the same trend in their 

 population fluctuations such differences in relative abundance occur frequently. Accordingly, 

 movements tending to equalize the distribution of the birds are continually taking place. 

 But. because the net changes in population are usually small, it is difficult to follow them 

 in detail. 



Although adjustments of this kind may occur at any time they are mainly associated with 

 the fall and early spring seasons. In fact the terms "fall shuffle" and "spring shuffle" respec- 

 tively have been used to describe them.* In general birds of the year are chiefly involved. 



The best indication obtained by the Investigation concerning the rate of spread into de- 

 pleted habitats came in connection with a series of population control studies on land 

 adjoining the Connecticut Hill study area. During the winter of 1933-34, 34 grouse were 

 collected from coverts totalling 681 acres of grouse cover, reducing the number present to 

 four birds by April. One month later 12 others had moved in and by the end of August 

 further movements had increased the number of adults to 29, a gain of 25 in five months. 



The following winter, similar collecting was done in another covert of 572 acres. By March 

 1 there were 13 grouse left. During the ensuing month 13 more were taken but the population 

 in early April was estimated at 17. Two additional birds were shot on April 7 and two others 

 were lost to natural enemies during the spring reducing the number left to 13 by the end 

 of May. Apparently none had moved in since March. By August 31 the estimated adult 

 population had risen to 21. representing a total influx of at least 25 birds in six months. 



In 1935-36, the population of a third tract of 273 acres was reduced from 22 birds in Oc- 

 tober to two in March. But in the interim, 24 birds had been taken. Thus four birds, in 

 addition to those present at the start, must have moved in during the late fall and winter. 

 Five more moved in by the middle of April, one more in May and one more in the summer. 



As the collecting carried on in these experiments was not begun until after the period of the 

 "fall shuffle", these records demonstrate that apj)rociable shifts to restock depleted coverts 

 may occur in March and early April and show that summer movements also take place. The 

 consistency of the results on all three units indicates that such movements are customary and 

 that any unoccupied territory in good grouse range will be quickly found by roving birds 

 except possibly during times of widespread scarcity. The distance from which these inflowing 

 birds had come is not known although in most instances they at least had to negotiate stretches 

 of open land from a hundred yards to a quarter mile wide. That such distances are no bar- 

 rier to grouse has been noted in the discussion of ni<il)ilit\". It is probable that many of 

 the birds had traveled iiuicli farther than simply across the open land extending to the next 

 covert. 



Carrying Capacity 



The ability of game range to support wildlif<' is limited. Like any container it can be 

 filled only so full. Surpluses spill over and must find accommodation elsewhere or be elim- 



• Src Ch.i.lfr V. p. 255. 

 A ge<< Chaptrr V. p. 25.1. 



