130 PROCEEDINGS OF THE ACADEMY OF [April, 



species vary in their manner of arrival in different years, being con- 

 centrated one season and scattered in another. 



The proper study of fluctuations in the numbers of each species at 

 any point, such as would warrant an estimate of. bulk arrival, requires, 

 except in a few cases, far more time than the majority of observers can 

 possibly give to the work — if indeed the task is possible for one indi- 

 vidual — and consequently where such an estimate has been attempted 

 the personal equation enters to such an extent as to render the results 

 of little value. 



It would seem that, with the comparatively small amount of time 

 at the disposal of most observers, it would be better to suggest the 

 recording of such occasional "bulk arrivals" as are so marked a feature 

 of the migration as to become obvious, rather than to ask for a record 

 of this sort for each species, which must from the nature of the case be 

 in the vast majority of instances an estimate. 



At the same time, however, the date of the first arrival, often an early 

 straggler, does not in itself give us a proper record of the migration of the 

 species, and it is here that the combination of a number of local records 

 proves invaluable and furnishes a far more accurate resume of the 

 flight of the species than can possibly be obtained by any individual 

 observer. 



For instance take the arrival of the Wood Thrush in the ten-mile 

 circle about Philadelphia in the Spring of 1906. Thirty-one observers 

 recorded it as follows : One on April 25, two on April 28, ten on April 

 29, five on April 30, eight on May 1, and one each on May 2, 3, 4, 10 

 and 12. This record obviously warrants us in saying that for this 

 area pioneer migrants arrived on April 25 and 28, while the bulk of the 

 migration occurred from April 29 to May 1, after which date it was 

 impossible, on account of the presence of the bird at almost all points, 

 to judge how much further transient migration was in progress. The 

 dates upon which the "first arrivals" are massed are obviously the 

 dates upon which the "bulk" arrived. The late dates are to some 

 extent due to failure on the part of the observer to be in the field on the 

 day on which the species first arrived, but in part they represent actual 

 absence of the species from these particular localities, as it is a matter 

 of record that on several occasions a species has been seen regularly 

 for some days at one locality before a single individual has appeared 

 at another station nearby, in spite of careful search at the latter place. 



The actual progress of the arrival of the Wood Thrush in 1906 within 

 the Philadelphia ten-mile circle may be shown more graphically in the 

 accompanying diagrams. 



