Sex Ratios in Pigeons. 
505 
maximum time any birds sat was 28 days,* whereas the mean period 
which they will sit on the eggs in case they do not hatch, as deter¬ 
mined from these 59 records, is 22.69 days. There is therefore 
a tendency for domestic pigeons, as represented by this group, to 
incubate the eggs not simply to the mean (17 days), nor indeed to 
the maximum (20.5 days) time of hatching, but for a somewhat 
longer period, the average being practically 23 days. The incubating 
instinct is not, therefore, one exactly adjusted to the number of 
days required for the young to hatch—such an exact adjustment 
would in fact be out of the question owing to the considerable varia¬ 
bility of this period—but there is rather, as in many instincts and 
other biological adjustments, a factor of safety. Physiological processes 
in general are characterized by variability, and nature, rather than 
drawing the line too closely, makes allowance for the extremes. In 
the present instance the factor of safety may be considered as 6 days, 
the difference between the mean time of hatching of the second egg 
and the average time the birds tend to sit. This is fully two days 
longer than necessary to hatch the eggs which required the longest 
time (20.5 days) according to our records, and if we may generalize 
from our data, fully 68% of the birds sit long enough to hatch even 
the few eggs that reach this limit. 
The foregoing data would appear to show conclusively that Raspial 
was wrong in attributing to the domestic pigeon an exact “notion of 
the time necessary for incubation” or anything comparable to it. 
For some years the senior author has desired to test the matter on 
a wild species, but has had little opportunity to do so. In the spring 
of 1914, however, with the assistance of Mr. A. R. Cahn, a complete 
record was made of the laying and incubation of a pair of Mourning 
Doves (Zenaidura macroura carolinensis ). The nest was some 12 or 
15 feet from the ground on the horizontal branch of a tamarack tree 
on the campus of the University of Wisconsin. The chronological 
history of the case is as follows: 
*The statement of a maximum of 30 days in a preliminary communication (Cole, 1911) was based 
on the time of laying of the first egg rather than the second. 
