^°19?8^^] Recent Literature. 489 



accurate for comparative work are to be obtained. This, I think, is to be 

 found by securing a large number of observers in a limited area and by 

 combining their results, as has been done by the Delaware Valley Ornitho- 

 logical Club in the vicinity of Philatlelphia. If we had seven-year records 

 kept by thirty-five individuals within ten miles of Washington, and a 

 similar series within ten miles of Boston for comparison with the Phila- 

 delphia series, then I think we should be able to estimate with some degree 

 of accuracy the progress of migration between these points." 



Mr. Stone's discussion of 'Waves and their Components' is of special 

 interest. It is based on the records of forty-seven common species, for 

 which the data are fullest, and covers the years 1904-1907. He formally 

 tabulates eleven waves, but gives the number as varying from eleven to 

 fourteen for the spring migration, to the consideration of which the present 

 discussion is restricted. He finds "a remarkable correspondence in the 

 species which make up each wave. And the same 'wave' may be recog- 

 nized through a number of years by its component species, though its 

 date may vary considerably. ' Sometimes a movement may be interrupted 

 by unsuitable weather and be resumed again later, making two apparent 

 waves in one year which correspond to one in other years .... It seems 

 then that certain species migrate together, advance stragglers of some 

 accompanying the bulk movement of others, and that each species is 

 ready for migration at approximately the same time each year, the exact 

 date depending upon a favorable combination of meteorological conditions." 



Mr. Stone's paper concludes with a tabulated summary of the arrival 

 dates of 90 species for the years 1902-1907, based upon the records of 25 

 to 35 observers for each year, "all located within ten miles of the center 

 of Philadelphia." The table gives under 'first arrival' the average date 

 and extreme dates for the whole period, and under 'bulk arrival' the date 

 for each year and the average date for the whole period. It is noticeable 

 that the length of the interval between average 'first arrival' and 'average 

 bulk arrival' varies markedly in different species. Thus, for 70 species 

 for which the record is complete, the interval between 'first arrival' and 

 'bulk' is from 4 to 8 days in the case of 45 species, 9 to 15 days in the case 

 of 19 species, and 16 to 30 days in the case of 6 species. The last 6 are all 

 early migrants, and to some extent winter residents. 



A vast amount of patient and painstaking labor, continued through 

 many years, has been expended by Mr. Stone to reach the results here so 

 clearly presented, showing that his interest in the subject is both deep and 

 lasting; and he has well earned the hearty thanks and congratulations of 

 all his fellow-workers in this field of research. It is to. be hoped that his 

 work and that of the other members of the Delaware Valley Ornithologi- 

 cal Club will incite other local ornithological associations to similar effort. 



We are not to infer, however, that all previous work on bird migration 

 by individual observers has been in vain, antl that our general conceptions 

 of the average date of the spring arrival of birds based on the observations 

 of isolated (as opposed to organized) observers is to any great extent 



