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A Study Of The Incubation Periods Of Birds
Dr. Bergtold bravely attacks a problem which has ever aroused the interest of ornithologists, by assembling all the available facts and theories relating to it. This evidence, with the many attempts which have been made to determine its significance, he weighs carefully, and finally presents his own conclusions.
The authentic incubation periods recorded vary from ten days in the Cowbirds, Bobolinks, certain Warblers and some others, to as many as fifty-six or fifty-eight days in the Emu and sixty days in the Black-eye-browed Albatross.
To account for this wide difference in time is the object of Dr. Bergtold's studies. Lack of satisfactory data concerning many of the factors involved prevent him, he writes, from presenting other than tentative conclusions.
While admitting that the incubation period "is loosely related to the size of the species, and still more loosely to the size of the egg," Dr. Bergtold believes that this period is primarily determined by thetemperature of the bird. The existing information in regard to the temperature of birds indicates that the 'lowest' forms (that is, those most nearly related to their reptilian ancestors) have the lowest temperatures (averaging 100Â° - 104Â°) while the 'highest' birds have the highest temperatures (averaging 106Â° - 110Â°). Here, apparently, we have a physiologically reasonable explanation of why the higher birds (Thrushes, Warblers, Finches, etc.) have the shortest incubation periods.
Dr. Bergtold has assembled more information concerning this fascinating and important subject than has before been available in a single volume, and his work marks the starting-place for all future investigators of the phenomena he so ably discusses.
-Bird-Lore: An Illustrated Bi-monthly Magazine, Volume 19 
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