ROLE OF PREDATION 331 



diseased birds usually retain their vigor up to a very short time prior to death. On the 

 other hand an earlier decline usually takes place in cases of heavy parasitism. 



Physical fitness may also be affected by other influences. It has already been pointed out* 

 that during the early brood period grouse chicks regularly suffer considerable losses from 

 causes mainly other than predation. Undoubtedly the vulnerability of these birds is greatly 

 increased at such times and predators frequently become the ultimate executioners. Then too 

 there is some indication that certain combinations of weather conditions may lower the vi- 

 tality of older chicks and thereby render them more easily caught. Similarly weather may 

 decrease the resistance of adults during the late winter''. There has been no indication, how- 

 ever, that starvation is of importance. 



Nevertheless, grouse, thoroughly up to par physically, become more susceptible to predation 

 under some circumstances than others. An outstanding factor influencing such variations is 

 the shelter value of the habitat. The availabiUty of escape cover is of the utmost importance 

 in enabling birds to elude their natural enemies. Since in any covert there is a limit to the 

 amount of such cover, surplus individuals become increasingly vulnerable as ihey are forced 

 to occupy less favorable territories. This was well demonstrated in the instance already men- 

 tioned^ in which one section of Connecticut Hill, after attaining the excessive fall density of 

 a bird to 2.5 acres, still suffered a high loss the following winter. 



There is also evidence that during the winter birds of the year are more vulnerable than 

 older individuals, both as a result of inexperience and because the better territories tend to 

 be already occupied. At times snow-roosting birds may be more easily captured by certain 

 predators, but at other times this habit may have just the opposite effect. 



Even under the best conditions the vulnerability of individual grouse is constantly chang- 

 ing as they pursue their daily routine of activity. Among the chicks, particularly during the 

 first few weeks of life, the relative "mothering" ability of the female undoubtedly is of great 

 importance. Similarly, circumstances which cause a chick to peep increase its chances of 

 being located and caught. 



Significance of Predation 



In the introduction to this chapter it has been pointed out that predation serves a very 

 necessary function throughout the organic universe in helping to keep the numbers of all 

 species within limits compatible with the environmental niches which they occupy. So it is 

 with respect to grouse. Yet it may seem to the reader, particularly the sportsman, from the 

 foregoing discussion of its effect in New York that predation is exerting an undue pressure 

 on this game bird. 



A fact often lost sight of, however, is that adult grouse exhibit an intolerance of population 

 densities above about one bird to four acres even in the best coverts. Like Daniel Boone, 

 who it is said at one time moved farther into the wilderness when a family settled within 

 some 20 miles of him, these birds tend to avoid crowding beyond that level. In other words, 

 predators or no predators, surpluses over this saturation point may be expected to disperse 

 of their own accord after the birds of the year have "come of age" in the fall. 



But most coverts are not of sufficiently high quality for this characteristic to be a regular 

 influence. Yet this merely means that even lower limits are imposed in such cases depending 

 on the nature of the cover*. Fall surpluses above this level tend to be eliminated by over- 



* See discussion of Role of Predation During Brood Period, p. 316. 



A See Chapter VI, p. 304. 



t See discussion of Predator Pressure, p. 327. 



t See Chapter XII, p. 523. 



