526 



PRODUCTIVITY OF C ROUSE POPULATIONS 



of the Investigation this has been of little significance with respect to the productivity of 

 ruffed grouse populations. Females that have more than just commenced incubation seldom 

 undertake to renest, while for those that do the average clutch size is smaller and the degree 

 of infertility higher than among first nests*. Moreover, the great majority of the unsuccess- 

 ful nests recorded had been destroyed during the latter part of the incubation period. Further 

 evidence that renesting has not been important is the fact that the nest losses observed 

 largely account fi)r the differential between the ininiher of females in the breeding popula- 

 tion and the number of broods ultimately materializing in the area. Therefore this influence 

 has not been considered strong enough to warrant separate recognition in the life equations 

 presented later in this chapter. 



A SUCCESSFUL CROUSE .NKM i;i.iui;i. AM) .\ri l.K llATtlllM. 



A I' finrrou 



The principal cause of nest mortality has consistently been predalion. A number of other 

 sources of loss have been recorded but altogetlu-r they have accounted for only aliout one 

 out of ten broken-up nests, or four out of 100 clutches laid. The instances of this kind which 

 have been observed are listed in table 80. 



In addition to losses resulting directly from nest destruction, jiroductivity is also reduced 



• Sec Chaplrr Mil. p. 3f>4. 



