Producti\aty and Populations 289 



see in this record any indication of a mishap hkely to cause an ab- 

 normally low productivity. However, even a twelve-year record 

 may not bring out such a relation, if it exists, particularly since no 

 catastrophic declines occurred in the grouse populations in New 

 York during that period. 



Eggs that die during incubation are identified by the presence 

 of the embryo when examined after hatching time. Most of these are 

 birds that fail to hatch properly, or in time, and die at full develop- 

 ment. However, as already noted, any cases of dead germs in the 

 very early stages of incubation would be misidentified as infertile. 

 Therefore, the figures given for losses by embryo death are possibly 

 a little lower than actual. The average proportion of embryos that 

 die during incubation, as determined from the New York records, 

 is about two and one-half per cent and the annual figures varied 

 from two-tenths per cent to five and nine-tenths per cent. Here, as 

 in the case of the infertile egg record, there is no regular trend in- 

 dicated, and the losses do not constitute a serious threat. 



Dead embryos may result from either external influences, as low 

 temperatures or too long exposure when the bird is off the nest, or 

 an innate weakness of the germ. Since the latter cannot be iden- 

 tified as a cause of death, the tendency is to attribute the losses to 

 exposure. The fact is we do not know the answer. Shifts in per- 

 centage of embryo losses do not correlate with recorded weather 

 data, tending to indicate that some of these losses may well be due 

 to inherently weak germs ( data derived from IV. Y. S. Cons. Dept. 

 Ann. Reports). 



Longevity and Breeding Years; Incubation Period. The full po- 

 tential span of life for grouse is rarely, if ever, attained in the wild 

 and never has been determined. In captivity, grouse have been 

 known to live for many years, but here too they rarely live long 

 enough to die from natural body degeneration. In the face of all 

 the hazards facing a grouse in its life, the potential life span is un- 

 important. Judging from the character of the bird, it probably can 

 live a decade or more. 



The actual life span of grouse depends upon when its time is be- 

 gun. If we count from the time of hatching, the average length of 

 life is a very few months; if we begin counting only with birds that 

 reach maturity, the span is about twelve months; if we count only 



