^ook j@eh)£( antr Iftebtetas; 



A Study of the Incubation Periods 

 OF Birds: What Determines Their 

 Lengths? By W. H. Bergtold, M.D., 

 M.Sc. The Kendrick- Bellamy Co., 

 Denver, Colo. 8vo. 109 pages. 



Dr. Bergtold bravely attacks a problem 

 which has ever aroused the interest of 

 ornithologists, by assembling all the avail- 

 able facts and theories relating to it. This 

 evidence, with the many attempts which 

 have been made to determine its sig- 

 nificance, he weighs carefully, and finally 

 presents his own conclusions. 



The authentic incubation periods re- 

 corded vary from ten days in the Cow- 

 birds, 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-eyebrowed Albatross. 



To account for this wide difference in 

 time is the object of Dr. Bergtold's 

 studies. Lack of satisfactory data con- 

 cerning many of the factors involved pre- 

 vent 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 the 

 temperature 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 ioo°- 

 104°) while the 'highest' birds have the 

 highest temperatures (averaging io6°- 

 110°). Here, apparently, we have a 

 physiologically reasonable explanation of 

 why the higher birds (Thrushes, Warblers, 

 Finches, etc.) have the shortest incuba- 

 tion 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 



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future investigators of the phenomena he 

 so ably discusses. — F. M. C. 



The Way to Study Birds. By John 

 Dryden Kuser. With nine colored 

 plates by Louis Agassiz Fuertes. G. P. 

 Putnam's Sons, New York and London. 

 i6mo. xii -|- 85 pages. 



Somewhat over thirty years ago there 

 appeared a popular book entitled 'Our 

 Birds in Their Haunts.' It was followed, 

 a few years later, by such less ambitions 

 but possibly more effective guides to 

 bird-lore as 'Fifty Common Birds and 

 How to Know Them,' 'Our Common 

 Birds and How to Know Them,' 'Birds 

 Through an Opera-Glass,' etc. From that 

 day to this the subject of bird identifica- 

 tion has claimed the attention of many 

 authors, culminating (with discoveries in 

 color printing) in inexpensive pocket 

 keys and guides with excellent colored 

 figures of every species of bird known to 

 inhabit the area concerned. Neverthe- 

 less, with that fine enthusiasm for his 

 method of presentation, without which 

 most books would never have been born, 

 Mr. Kuser tells us, "For the beginner 

 to learn avian identification is at pres- 

 ent hopelessly complicated and unneces- 

 sarily discouraging, because of the require- 

 ment to wade ignorantly through a 

 heterogeneous mass of terms and descrip- 

 tions, or to go bird-hunting without the 

 remotest idea of what he may expect to 

 find most common or most easily identi- 

 fiable in the special locality in which he 

 searches." 



Mr. Kuser writes of fifty common land- 

 birds from "New York City as a center." 

 His method of arrangement is seasonal, 

 "twelve abundant permanent residents"" 

 being first treated. Then follows a chap- 

 ter on 'Note-keeping' and another on 

 'How to Use the Key' which is given at the 

 back of the book. Chapter V includes 

 "The fifteen most abundant Summer 

 Residents," and Chapter VI "The Fifteen 



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