362 REPRODUCTIVE CAPACITY OF THE SPECIES 



in its egg laying by physical inability to continue. On tho other hand, the smaller number 

 of eggs laid in renesting attempts does give evidence that thr liirrl rn;i\ then approach the 

 limits of its physical ability. 



A definite conclusion as to the cause or causes that bring an end to egg laying is not pos- 

 sible with our present knowledge but the evidence points to changes in light intensity as the 

 underlying factor, just as it is the stimulant that starts the production of eggs earlier in the 

 spring. 



Yearly Variations 



It has been noted that the changes in the number of eggs laid per female year by year have 

 not been such as to cause material differences in the following autumn pojjulatiDns. An exam- 

 ination of table 50 will reveal, however, that some variation did exist in different years. 



Comparing state-wide averages, the extreme variations were between 11.1 eggs in 1935 and 

 12.1 eggs in 1937. The maximum change from one year to another was the increase from 

 11.5 eggs in 1936 to 12.1 in 1937. 



One might expect greater yearly variations from the records of a more restricted area since 

 state-wide averages may tend to reduce irregularities. It is informative, therefore, to examine 

 table 51 wherein the figures have been derived by regions. 



TABLE 51. GREATEST VARIATION IN AVERAGE ANNUAL CLUTCH SIZE (IN EGGS) 



These gross variations are significant* but no consistent trends are discernible to explain 

 them. The Investigation has no evidence to show marked changes from year to year in the 

 egg laying of binls of llie same age. However, there seems to lie a definite tendency for birds 

 in their first year to lay fewer eggs than older ones! Thus the general average number of 

 eggs would be affected if the jiroportion of youngsters in the |)(>pulafion changed appreciably. 

 This actually is the case" and thus offers a means by which the observed yearly differences in 

 clutch size may have come about. These changes do not. however, seriously affect grouse 

 populations nor explain the great changes that take place in grouse numbers. 



Variation in Different Regions of New York 



The figures for the axcragc numlier of eggs per clul( h for the different portions of the State 

 are given in table .SO. The average for the Catskills and for the Connecticut Hill study area 

 is the same as the state-wide average. 11.5 eggs. The Adirondack nests averaged one-half egg 

 lower and those from "Rest of State" four-tenths higher than this. Most of the latter group 

 were taken from the southern tier counties in range similar to Comiecticut Hill. If these two 

 are lumped, the average for the dis<-ontie(ied covert range is 1 1.7 eggs per nest. The difference 



* By Annlvni* fil Varianrr. 

 A See Chaplrr XII. p. 513. 



