370 EXPERIMENT STATION EECORD. 



only a slight indication that more males than females from such broods survive 

 to adult life — placed at six months. 



"A consideration of the ratio of males to females in each of the age groups 

 does not indicate a high relative mortality of females in the ages preceding the 

 adult state. There is a high mortality of both sexes during the first two or three 

 years of their adult life, and this is especially high in the females between the 

 ages of one and two years. The higher mortality of females at early adult 

 ages, together with the higher proiX)rtion of males hatched (105:100), may be 

 in large part responsible for the prevailing notion of a considerable excess of 

 male pigeons in adult populations and seems to furnish real substantiation for 

 this notion. The fact that males are generally more easily recognized than 

 females probably adds to this impression. 



" The number of unisexual broods, in which the squabs are either both male 

 or both female, somewhat exceeds in our records the bisexual broods (one squab 

 of each sex), but the odds against the numbers obtained representing a poten- 

 tial equality are very slight. These facts aire directly contradictory to the 

 statements that the two eggs usually produce a male and a female squab. 

 Considering only the unisexual broods, the number of ' both males ' to ' both 

 females ' is practically equal. 



"A comparison of the numbers of each sex hatched from first eggs and from 

 second eggs, respectively, shows no tendency for the former to produce exclu- 

 sively males and the latter females, but as a matter of fact more males than 

 females are hatched from both. 



" The mean time of laying of the first egg is about 5 p. m., and of the second 

 egg about 1 o'clock in the afternoon of the second day following. The mean 

 interval between the laying of the two eggs is practically 44 hours. The mean 

 time between the laying of the first and second eggs decreases progressively in 

 the months from February to July, inclusive. There is a very sensible positive 

 correlation between the time of laying of the first and of the second egg. The 

 equation of the regression curve is given, which enables one to calculate the 

 most probable time of laying of the second egg when the time the first was 

 laid is known. 



" The mean time of hatching of the first egg is 16.5 days after the laying of the 

 second. The mean time of hatching of the second egg is 17 days after it is 

 laid. On the average, therefore, the time from laying to hatching of the first 

 egg is nearly a day and a half longer than it is for the second egg. This is 

 probably to be accounted for by the fact that the first egg receives very little 

 incubation until the second is laid. There is a high correlation between the 

 times of hatching of the two eggs of a clutch. 



" So far as the data presented go, they appear to indicate that sex in pigeons 

 is determined according to the laws of chance. 



" In case the eggs do not hatch they are seldom abandoned at the end of the 

 normal period of incubation, but the birds continue to sit on them for a time 

 longer. The length of time they will continue to incubate the eggs varies, but 

 averages practically six days after the normal period, making the mean total 

 time of incubation when the eggs do not hatch 23 days after the laying of the 

 second egg. This continuance of incubation beyond the normal time under 

 such circumstances constitutes a ' factor of safety ' in the incubating instinct. 

 A pair of mourning doves continued to sit on substituted eggs for four days 

 after their own had hatched in an incubator, thus disproving Raspial's assertion 

 that wild birds have an ' exact notion of the time required for the eggs to 

 hatch.' " 



References are appended. 



