552 PRODUCTIVITY OF GROUSE POPULATIONS 



While there can be little doubt that predators are the chief executioners of grouse, except 

 during the early brood period, this game bird can be considered little more than a delicacy 

 or dessert to most of them. At the same time a host of other influences are responsible for 

 fluctuations in the vulnerability of the birds. The effect of shelter quality has been noted as 

 has also that of the density of the grouse population. Weather, too. can be fayorable or un- 

 fayorable. Another highly important factor is the availability of buffers. Usually more nu- 

 merous and easier to catch, they constitute the staple diet of predators as long as the supply 

 lasts, even though grouse may also be plentiful. But let the buffers decrease substantially 

 and the fox. weasel, great-horned owl and goshawk, harder put to keep from starving, make 

 more frequent and more determined efforts to capture grouse. Thus an epizootic, decimat- 

 ing the ranks of rabbits and mice, may indirectly exercise a strong influence on partridge 

 abundance even though predator populations remain essentially unchanged. 



That predators are more likely to catch birds slowed down by disease there can be little 

 doubt. Though such cases do not appear to be as common as is generally supposed, the re- 

 moval of afflicted birds from the coverts obviously is beneficial in inhibiting a further spread. 

 In America, the usual restrictions on hunting are still so fixed as to discourage the taking of 

 a large part of the crop when appreciable surpluses occur. Thus, during the more produc- 

 tive years, disease- may actually be encouraged by leaving an over-abundance of birds in the 

 field. To the extent that predation serves to whittle down this excess, it may help to maintain 

 a goodly number of birds. So clearly is this situation recognized in Great Britain, where pred- 

 ators are scarce, that gamekeepers each year spend considerable time in "shooting down" 

 the population of Scotch grouse, left at the end of the hunting season, to a level where out- 

 breaks of disease are unlikely and the carrying capacity of the moors is not exceeded. 



Furthermore, reduction of a decimating agent is no guarantee of a corresponding in- 

 crease among the grouse. Compensatory adjustments soon tend to take its place. For ex- 

 ample, unless applied to a large area, the effects of predator control are rapidly offset bv 

 the influx nf individuals from surrounding territory. Similarly, as was observed on one study 

 unit, if only certain species such as foxes and weasels are removed the depredations of others 

 such as skunks and crows increase. On Valcour Island (N.Y.). where this relationship was 

 also studied, after the elimination of all the usually more important predators, the propor- 

 tion of nesting loss attributable to red squirrels increased. 



Even if some combination of circumstances should allow an excessive density of birds to 

 build up in the fall, heavy overwinter mortality may usually be expected to ensue unless 

 the majority of the surplus is taken by hunting. It seems probable, too. that the occurrence 

 of high concentrations may often set the stage for serious outbreaks of disease, \ature is 

 constantly trying to balance her books. 



One cannot examine this picture without becoming conscious of il> licnicndous complexity. 

 There is little solace in it either for the sentimentalist who would stop all hunting or for the 

 sportsman who thinks an abundance of birds can be assured through predator control, for the 

 data clearly show that Nature's grouse budget can never be increased for long by either 

 means. Certain factors, such as weather, accident and the productivity of the bird, are be- 

 \ond human control. ()\er others, such as hunting, disease, predation. buffer abundance, 

 food and shelter, the game manager may exercise varying degrees of regulation. Rut grouse 

 populations are dynamic, never stable. Experience indicates that their fluctuations are not 

 easily predictable, so that measures taken to allcyiatc scarcity or maintain abundance are of- 

 ten too little and too late. 



