48 PROCEEDINGS OF THE 



cases and differ by one day in the other five, which shows that 

 by the method of computing "bulk arrival" we are getting 

 remarkably accurate results. If we could only induce observers 

 clustered around some other large city, some distance from Phil- 

 adelphia, like Boston or Washington, to keep similar records 

 and tabulate them in the same manner, then we should be able 

 to estimate the progress of the migration and the actual speed 

 at which the birds travel. The data heretofore used is not suf- 

 ficiently free from error to warrant accurate results. One has 

 but to compare the ten-year averages in the last table for any 

 species at ten stations where from two to four careful observers 

 have kept the records, to see at once what variation they present 

 at stations only a few miles apart, while stations like George 

 School and Concordville, forty miles apart, along the line of the 

 Delaware Valley, and twenty-five miles different in latitude, 

 show no constant difference, five species averaging earlier at the 

 more southern station, two at the more northern and two the 

 same.^ 



' Cf. for discussion of this matter Stone, Proc. Acad. Nat. Sci., 1908, pp. 

 128-156. 



