248 BULLETIN 162, UNITED STATES NATIONAL MUSEUM 



33.59 millimeters. The eggs showing the four extremes measured 

 49.1 by 34.1, 45 by 35.3, 40.5 by 32.9, and 40.8 by 30.2 millimeters. 



The laying of the eggs as determined by studies of captive birds 

 extends over a period of days equal to nearly twice the number of 

 eggs in the set. Apparently approximately the same ratio holds for 

 birds living under natural conditions. In a nest in Adams County, 

 Wis., the first egg was found on May 5, 1929 ; on May 12 there were 

 5 eggs; and the set of 11 eggs was completed on May 22. The time 

 required to complete a set of eggs depends on a number of factors, 

 such as the condition of the weather, the health of the bird, and the 

 available food supply. The laying of the eggs is not necessarily 

 on alternate days, but more apt to be very irregular. Certain eggs of 

 the set are laid on successive days to be followed by a lapse of two 

 days before the next egg is deposited. 



During the laying period of one individual under observation 

 the bird covered the rather exposed eggs with nesting material before 

 she left them. This instinctive habit may be for the purpose of con- 

 cealment or for protection from extremes of temperature or for 

 both. 



The number of eggs layed by the prairie chicken based on studies 

 made of 40 nests, in which the number of eggs was presumably com- 

 plete, varied from 7 to 17. The average number was 11.5 eggs per 

 set. A nest containing 21 eggs has been reported, but such unusual 

 sets probably represent the eggs of two females using the same nest. 



The dates when the nests were found are of some interest to those 

 who may desire to know the probable date when a nest of the prairie 

 chicken may be found. Of 41 nests in which the date of finding 

 was recorded, 20 were found in May, 18 in June, and 3 in July. The 

 earliest date was of a nest found on May 5, when it contained a single 

 egg, and the latest was a nest found July 10 that still contained eggs 

 on July 15, the last time it was visited. The average date when the 

 41 nests were found is June 3. 



Judging from these results we may say the last week of May and 

 first week of June are the times when one may expect to find the 

 largest numbers of nests of the prairie chicken in central Wisconsin. 



Young. — The incubation of the eggs is performed by the female. 

 Soon after the courtship season the male goes into retirement and 

 undergoes the ordeal of molting. I have never seen the male bird 

 near the nest, nor have I ever observed him participating in the care 

 of the young. 



Incubation begins soon after the set of eggs is completed, but in 

 certain cases where we have found live embryos in eggs left in the 

 nest after the rest of the brood had departed it was evident that 

 incubation had started before these eggs were laid. Sometimes an 



