1889. ] STONE, Graphic Representation of Bird Migration. I41 
the number of species that occurred in the locality during that 
week. And a curve constructed from these results (Fig. 2, A) 
will show the variation in the number of species during the period 
that the investigations cover. By using the actual number of 
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T#rch iz, 
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ili sy 
Tle 
be sees 
Fig. 1. Portion of Chart showing the birds recorded at Germantown, Pa. Spring, 1888. 
species seen during each week, as recorded in the notebook, a 
curve may be constructed which will be quite similar though less 
regular, since the meteorological conditions here play an import- 
ant part. For instance, on a cold or rainy week the number of 
species observed will be less, simply because it was impossible to 
make many observations. A curve (Fig. 2, C) constructed on 
the average number of species seen per day, for each week, will, 
as stated above, show the variation in regard to both individuals 
and species. 
Looking at curve A, Fig. 2 or 3, representing the variation in 
the number of species recorded at Germantown, Pa., during the 
spring migration of 1888, we see the number at a minimum 
through the winter, until about the middle of February. At this 
time the first increase is noticed, caused by the arrival of the 
Grackles, Red-winged Blackbirds, Robins, and Bluebirds. Then 
the number is stationary until the second week of March, when 
a gradual increase begins which continues more rapidly during 
April and still more rapidly during the first week of May. The 
number finally reaches its maximum in the second week of that 
month and then decreases rapidly until it again becomes station- 
