LENGTH OF INCUBATION PERIOD 3 



EXPERIMENTAL RESULTS 

 Relation of Length of Incubation Period to Weight of Eggs 



Huggins and Huggins (1941) have recently reviewed the literature on varia- 

 tion in the length of the incubation period in wild birds. These workers are of 

 the opinion that fresh egg weight is an important factor in observed variability. 



Byerly (1934) and McNally and Byerly (1936) found egg weight to be an 

 important factor in variation in length of the incubation period. Heavier eggs 

 generally required a longer incubation period than lighter eggs. 



In the studies here reported, the mean weight of the first ten eggs laid during 

 the hatching season was used as a measure of egg weight. By setting this mean 

 against the emergent period of each chick from the respective dams, the correla- 

 tion between egg weight and length of incubation period was approximated. 



There were 430 dams that laid at least ten eggs and the mean egg weight of 

 these dams was tabulated against the emergent period of their 4730 chicks in a 

 correlation table. The constants arrived at were as follows: 



Number of dams 430 



Number of chicks 4730 



Mean egg weight of dams, grams 62.1 



Mean emergent period of chicks 4.2 (last third of 21st day) 



Coefficient of correlation —.0981 +.0097 



Regression was found to be strictly linear, so that the coefficient of correlation 

 may be used to measure association. Using either 400 or 500 degrees of freedom, 

 the magnitude of the coefficient of correlation is insufficient to be definitely sig- 

 nificant. It seems apparent, therefore, that this small negative value of the 

 coefficient of correlation indicates no relation between weight of eggs and length 

 of the incubation period in Rhode Island Reds. 



Relation of Hatching Date to Length of Incubation Period 



The range in hatching dates was limited, extending from March 7 to April 23. 

 All the data are thrown together in table 1 to indicate the percentage of chicks 

 emerging in the different periods as the hatching season advanced. 



The data indicate that the majority' of chicks hatched in periods 3, 4, and 5 

 which represent the second third of the 21st day, the last third of the 21st day, 

 and the first third of the 22d day. The number hatched on the last third of the 

 20th day was extremely small, and the number hatching at the end of the 22d 

 day was also small. 



In the hatches produced at the end of March and during the first half of April, 

 there were fewer early emerging chicks and a significant increase in the number 

 of chicks emerging on the 22d day of incubation. The first two hatches, produced 

 before March 15, agreed closely in the percentage of chicks emerging during the 

 different periods. A slight tendency was observed in the third hatch, produced 

 after the middle of March, for the chicks to emerge somewhat earlier than was 

 noted in the first two hatches. Since there was a tendency for the chicks in the 

 earlier hatches to emerge somewhat earlier than in the later hatches, this may in 

 part account for a more rapid growth in chicks from the earlier hatches during 

 incubation, to two weeks, and to four weeks, as reported by Hays and Sanborn 

 (1929). 



