FERTILITY OF THE EGGS 



365 



FERTILITY OF THE EGGS 



When a pair of healthy grouse that are each in breeding condition, mate at necessary inter- 

 vals, the eggs will be almost all fertile. Infertility may result where the hen bird is physi- 

 cally impaired, where mating is imperfect, or there has been no mating. 



The fertility record of the eggs in wild grouse nests observed are summarized in table 53 

 for those cases where all the eggs could be accounted for*. 



TABLE .53. INFERTILITY RATES IN FIRST NESTS AND RENESTS— ENTIRE^STATE 



—1931-1941 



The consistently high fertility rates in both first and renests are remarkable, especially when 

 one considers the complicities of the breeding cycle. The yearly variations have been consid- 

 erable — from one per cent in 1936 to 7 per cent in 1934. No evidence of trends from year 

 to year that might indicate a relationship to population changes are discernible. Likewise, 

 in no year was the frequency high enough to constitute a major threat. 



The average rate of infertility for 506 first nests was 2.6 per cent while for 66 renests it was 

 4.3 per cent. The rate of infertility in renests, while proportionately much higher than for 

 first nests, is still not of any great significance. The 1.6 per cent increase obviously is not a 

 large factor in grouse production but it does indicate a tendency for late breeding to induce 

 more infertility than is normal among first nests. 



There is, then, no evidence in this 11-year record that infertility is a very big factor in 

 determining grouse numbers. 



VIABILITY OF EGGS 



The embryo of a fertile egg is a living organism that may at any time during its develop- 

 ment period be subjected to adverse influences and, as a result, may die. Any one or more 

 of several causes may bring death. Among the more common causes may be listed inadequate 

 incubation, an inherited weakness in the germ, or some structural failure in the egg itself. 

 The cause of death in any particular instance cannot be identified except in rare cases. 



The records for 462 first and 64 renests for which data are complete are summarized in 

 table 54. 



* When the shell remaina plus the unhatched eggs failed to eqiuil itie [iiu>\vri rlulili nuinher. tlie record was not used in the 

 tables on infertility and embryo mortality of the eggs. Records from wild eggs that were collected and placed in incubators at 

 the Research Center were felt to be representative of fertility and are included in the tabic. 



