24 
duration to start migration. The notes received came from places where 
the southerly winds were felt, principally in northeastern Illinois, with 
the single exception of a batch of records from central lowa, where a 
northwest wind prevailed. The weather over the whole of the district 
was clear, with an average barometric pressure of about 30 inches. 
In western Nebraska and eastern Colorado the barometer, though 
high, was falling, and during the daytime of May 9 it fell rapidly. At 
Yankton, during the sixteen hours previous to 11 p. m., May 9, the press- 
ure fell .34 inch to 29.74inches. Thus the low area developed right in 
the Mississippi Valley, and its influence was speedily felt, bringing on 
southerly winds and sending northward almost the last great wave of 
the spring migration. The temperature did not vary much from that 
of the night before, except to rise a little in the immediate ‘vicinity of 
the low area and to fall at La Crosse and northeastward; the sky was 
mostly clear; the wind variable—the low area not having had time to 
fully affect the winds. All the records came from places where the 
winds were southerly—consequently from the cloudy places—so that 
although the larger part of the Upper Mississippi region was clear, the 
cloudy records form 73 per cent. of the whole number. Another thing 
was noticeable, namely, that notwithstanding the fact that the sky was 
cloudy, yet the dew point was many degrees below the temperature, 
showing that the air was very dry. The average of the difference be- 
tween the temperature and dew point of the records of May 5 is only 
four degrees, with a range from 0° to 8°, showing that, the air was al- 
most fully saturated with moisture. The records of May 11 show an 
average difference of fifteen degrees, with a range from 11° to 33°. 
Thus it will be seen that the humidity of the atmosphere has little or 
no effect on migration, and can be left out of future investigations. 
This wave of the night of May 9, like the one of five days before, was 
very extended. Along the western shores of Lake Michigan, where the 
weather had been cold and disagreeable for the four days previous, it 
was most strongly felt. Then little or no movement was noted until 
the Mississippi was crossed. Here, from Keokuk to Moorhead, the 
night was marked by great activity, and the movement extended ina 
southwesterly direction as far as southern Nebraska and Kansas. No 
notes were sent from southern Dakota, and it is probable that almost 
no migration took place in that Territory, as certainly none did in Man- 
itoba. Nor was the movement of special importance south of Keokuk. 
The influence of the low area had not yet extended south of that point, 
and it was not until the next night that a full bird wave occurred at 
Saint Louis. The above is an excellent example of a bird wave and a 
warm wave both working from the north southward. 
Recapitulating, in the same manner as was done for the March notes, 
it is found that the temperatures at which migration was made are as 
follows: At 46° there were 29 records of firsts; at 52°, 11 records; 559°, 
116 records; 59°, 66 records; 63°, 70 records; 67°, 9 records. Thus, in- 
