290 The Ruffed Grouse 



birds that reach a breeding season, the average lifetime extends to 

 about eighteen months. 



The number of breeding years for either sex is not known, but all 

 indications are that it is the full life span. Surely, it is the full actual 

 life of all wild birds. 



The normal incubation period is twenty-four days. This may some- 

 times vary slightly, usually by being lengthened. This abnormality 

 may result from excessive periods of time off the nest, which cumula- 

 tively result in a measurable extension of the full incubation period. 



Ratio of Females Nesting. For the hen grouse, taken as an individ- 

 ual, it is normal to breed, nest, and lay a complement of eggs each 

 year. When we consider all female grouse, however, we find that 

 there are occasional exceptions. We might expect a few grouse to be 

 miable to locate males in marginal grouse range or in poorly popu- 

 lated range; but this apparently does not constitute the full story 

 of failure in breeding of wild grouse. Why some grouse do not breed 

 when there is an adequate number and distribution of cock birds we 

 do not know. We can only surmise that there are occasional indi- 

 viduals that are physiologically or socially out of balance, as among 

 humans. 



Determination of the nesting ratio is di£Bcult, even as an ap- 

 proximation. Theoretically, the number of successfully nesting fe- 

 males {i.e. the number of broods), plus the number of broken-up 

 nests, less the number of renest broods equals the total number of 

 females when breeding and nesting are perfect. The nesting ratio 

 can, therefore, be derived from the formula: 



Per cent nesting females = 



(no. broods -f- no. broken-up nests — renest broods) X 100 



no. females 



With a good census, the number of females and the brood data 

 may be determined with accuracy. The di£Bculty arises in obtain- 

 ing the number of destroyed nests. Even the most diligent search 

 with trained observers will not bring complete success on a large 

 area. With our figure for destroyed nests too low, it is clear that 

 the nesting ratio will be lower than is actually the case. We can 

 estimate this discrepancy ff we may assume that the broken-up 

 nests not found are in the same ratio to those found as are the un- 



