106 MICHIGAN SURVEY, 1908. 



these evidences were present, but usually only one or two were recog- 

 nized, the most pronounced of which was the great increase of birds 

 as they passed along the roads from one clearing to another. 



In some cases the bird wave marked the date of first arrival, at other 

 times it simply marked the an-ival of the bulk. The bird waves were 

 generally sharply defined, so that their relation to the atmospheric 

 changes could be noted to the best advantage. The bulk of the migration 

 took place during the nights of bird waves, although there was a con- 

 stant going and coming of certain species throughout the fall. Being 

 almost constantly in the field the writer had excellent opportunities to 

 study the migration when it was most pronounced, that is, during the 

 large waves. 



1. First Wave. The first wave observed occurred on August 23, and 

 consisted principally of Tennessee Warblers, immature and adults being 

 about equal in number. At 7:00 A. M., the temperature was 58°, a 

 rise of 6° in the last twenty-four hours. The weather was clear, and 

 it was in fact one of the most beautiful autumn days of the season. 

 A light northwest wind prevailed, the barometer standing at 29.9 inches 

 (low). This wave lasted for three days. The day previous the weather 

 conditions had been about the same, except that the thermometer stood 

 at 52°. An area of low pressure (29.75) was advancing towards the 

 island and on this date was central over Assiniboia. On the 23d this 

 aren was central over Isle Royale and the area to the north and northeast 

 of it. On August 24 a low area was centered over northeastern Missouri 

 Avliile the high pressure which followed it reached very nearly to. Isle 

 Eoyale, thus lowering the temperature to 53° with a northwest wind and 

 barometer reading of 30.2. On the 25tb of August the high (30.3 inches) 

 included the island and the ai'ea directly south of it. The weather was 

 clear, no wind, and a fall in temperature to 50°. On this date the 

 beginning of the large wave of Blackpoll Warblers commenced, many 

 large flocks appearing before nightfall. On the fourth day of the wave 

 (August 26) the barometer fell to 30.2 with an increase to 54° in tem- 

 perature. There was scarcely a breeze, and the day was for the most 

 part humid and cloudy. The bulk of the Blackpolls arrived during the 

 previous night and throughout the day. The wave lasted for several 

 days, decreasing gradually in volume, so that it was difficult to tell ex- 

 actly when it stopped, if in fact it did not grade off into the next one. 



2, Second Wave. On August 30 great flocks of Palm Warblers in- 

 vaded the island. On the previous day the area of low pressure was 

 central over the region a little to the west of Isle Eoyale, with a bar- 

 ometric pressure of 29.9, a temperature of 61°, and wind northeast with 

 rain. On the 30th the low area had passed on to the St. Lawrence Val- 

 ley and the advancing high pressure was over Manitoba. The barometer 

 stood at 29.8 with a northwest wind blowing 4 miles per hour and a 

 temperature of 56°. This wave also lasted three days and might have 

 continued longer but for the severe gales which set in on the night of 

 September 1. 



The second day of this wave (August 31) the area of high pres- 

 sure (30.1) was central over all of Northern Michigan, a considerable 

 area north of it, and south to southern Wisconsin. "VS'ith the high pres- 

 Bure came a drop in temperature to 46°. The day was clear with a north 



