488 Recent Literature. ies 
RECENT LITERATURE. 
Bergtold on the Incubation Periods of Birds.'— It is refreshing to 
find an ornithological writer who strikes boldly out upon an essentially new 
line of research as Dr. Bergtold has done in the volume before us. The 
problem to the solution of which he has applied himself is by no means an 
easy one. The personal collection of the necessary data being out of the 
question, the author has had to depend upon such as could be compiled 
from widely scattered literature, the inaccuracies of which as the author 
explains have often proved confusing. When we realize that Dr. Bergtold 
was removed from any of the large scientific libraries and that his investiga- 
tions were “carried on in the spare moments of a fairly busy professional 
life,” we are astonished at the completeness of his treatment of the subject. 
To use his own words: ‘The problem in hand is to answer the question, 
Why does a house finch’s egg take fourteen days to hatch, an ostrich’s 
forty-two days, an emu’s fifty-six days, or a hummingbird’s fourteen days? 
....to analyze the published data,....to examine the explanations 
heretofore given....and to determine if there be a law which controls 
the length of incubation.”” The author considers in order the various 
possible factors which might affect the time of incubation, quoting previous 
writers and weighing the evidence for and against each theory, referring 
constantly to accompanying tables of length of incubation for various 
species, weight of birds, weights of eggs, and bird temperatures — all of 
them compiled from a wide range of publications and from some original 
data secured by the author and his friends. 
Dr. Bergtold tentatively concludes on the one hand that the length of 
incubation is only loosely related to the size of the bird or the egg and not 
at all to the longevity of the species, the body-weight egg-weight index, 
age of the female or size of the egg yolk. On the other hand he considers 
that there is a true length of incubation which is a deep seated, persistent, 
specific character, that bird temperatures are closely related to taxonomic 
lowness or highness of the species and finally that ‘‘a bird’s temperature 
determines or fixes the time of its incubation period,” — the higher the tem- 
perature the shorter the period. 
Dr. Bergtold further considers it likely that variation in the period of 
incubation and in temperature exists among the species of any family, 
from those taxonomically lowest to those taxonomically highest, so that a 
curve of variation from the lowest to the highest birds would show undu- 
lations within each family. In this connection he says: ‘‘The question 
of lowness or highness in birds, in the present discussion, is a question of 
1 A Study of the Incubation Periods of Birds. What Determines their Lengths? By W. 
H. Bergtold, M. D., M. Sc., Member of the American Ornithologists’ Union. The 
Kendrick-Bellamy Co., Denver, Colorado. 1917. 8vo, pp. 1-109. $1.50 postpaid. 
