1908.] NATURAL SCIENCES OF PHILADELPHIA. 139 



day by five observers, and by plotting the daily totals a chart was 

 obtained representing the fluctuations of the migration, which was 

 shown to correspond to rises and falls in the curve of temperature 

 variation for the same period. In my Birds of Eastern Pennsylvania 

 and New Jersey, 1894 (p. 28), a like method was employed. 



Similar and probably much more accurate results may be obtained by 

 plotting a curve based upon the total "first arrivals" within the ten- 

 mile circle as reported by our Philadelphia migration corps for each 

 day of the Spring. 



In the following diagrams such curves are shown for the years 1902 

 to 1907, accompanied by curves of temperature variation based upon the 

 mean daily temperature at Philadelphia as recorded by the United 

 States Weather Bureau, together with an indication of the days upon 

 which rain or snow fell. For this meteorological data I am under obli- 

 gations to Mr. T. F. Townsend, Director of the Pennsylvania Section, 

 U. S. Weather Bureau. 



In the early part of the season it will be noticed that "waves" of 

 migration follow closely after marked rises in temperature, but later on 

 at the height of the May migration the great "waves " or " rushes " often 

 occur without any corresponding temperature increase. 



It is well known that birds do not start to migrate on a rainy night, so 

 that it is natural to expect sudden drops in the migration curves to be 

 correlated with spells of rainy weather, and such is often the case. 

 Inasmuch as birds are sometimes overtaken by rainstorms after 

 starting on a clear evening, they often arrive at a locality simultane- 

 ously with the rain, and as. it is not possible to indicate in the diagrams 

 the exact time and extent of the daily precipitation allowances must 

 be made for some apparent discrepancies in this respect. 



In the following diagrams the vertical lines represent the days from 

 February 15 to ]\Iay 18, while the horizontal lines denote five degrees 

 difference in the temperature curve and ten units difference in the 

 migration curve; a unit in the latter curve being a "first arrival" record 

 at some one of the stations within ten miles of Philadelphia. Thus if 

 the migration curve reaches ten on a certain day it means ten first 

 arrivals, i. e., one species recorded for the first time at ten stations, 

 two species at five stations each, or ten differezit species each recorded 

 at a single station as the case may be. Periods of rainy weather are 

 indicated by the broken line immediately below the diagram, marked 

 "rain." Each migration is divided into two sections placed opposite 

 to each other, so that the curves run across both pages, with the 

 comments below. In each chart the upper curve represents tempera- 

 ture variation, the lower migration. 



