158 
EIGHTH REPORT. 
species on the second of May (1905), or a total of 166 species for the 
twelve years. The waves for the second week in May occur for five 
years only, with the total of twenty-six species, while the third week 
generally finishes the spring migration, and there are only three years 
of migration waves with a total of twenty species. These records show 
that the hardier birds, like the Robin, Bluebird, Meadow Lark, Black¬ 
bird and many other early migrants do not come at a stated time each 
year, but are governed in their migration to a certain extent by favor¬ 
able weather and food conditions. 
These records also show that certain later migrants tend to arrive 
in a given locality at about the same time each year, as for example, 
the Boblink arrived during the first week of May for nine years; the 
Palm Warbler has ten first arrival records for the same week, and the 
Myrtle Warbler, seven. Kirtland’s Warbler has been taken only four 
times at Ann Arbor—on May 14, 15, 16 and 18—which shows a won¬ 
derful regularity, while the last migrants, like the Blackpoll, Wilson’s, 
Mourning and Connecticut Warblers may be expected from May 15 to 
20. The weather conditions hastening or delaying them for only a few 
days, while the earlier migrants have a range for arrival of weeks, 
.(and in some cases months), due apparently to weather conditions. In 
connection with this phase of bird migration it would be interesting to 
study the weather maps for the past twenty-five years in order to see 
how closely the bird waves follow the favorable weather conditions. 
There can be little doubt but that the birds take advantage of the condi¬ 
tions that seem to be the most favorable for their successful migration, 
and enough study of the weather maps has been done at this Museum to 
show that “bird waves” can be predicted with some certainty. The Table 
of Species shows that many arrive here with remarkable regularity, ir¬ 
respective, of the bird waves. For instance, the Phoebe has seven arrival 
records for the fourth week of March which has no recorded bird wave; 
the Brown Thrasher has seven first arrival records for the third week 
in April (which has records for four bird waves in four years), only 
two of which this species took advantage; the Baltimore Oriole has 
twelve arrival records for the fourth week in April, which has eight 
waves, only three of which is used; the Blue-gray Gnatcatcher is another 
with eleven first arrivals, but came on only two of the eight waves; and 
the Yellow Warbler has ten first arrivals for the same week, and it 
came on only three waves. The first week in May has bird waves for 
twelve years", and of these the Whip-poor-will took advantage of but 
two,. although it has six arrival records for that week. The Bobolink 
with nine arrival records for the same week came on seven of the twelve 
bird waves, and the Palm Warbler, with ten arrival records, used nine 
of the twelve waves. The Chestnut-sided Warbler has only six arrival 
records for this week, but came every time on a wave. While the Black- 
throated Blue Warbler with twelve arrival records came on only three 
of the twelve waves. These instances refer, of course, to only the first 
recorded arrivals. The bulk of the species may have arrived on a bird 
wave, as it is a well-known fact that the first arrivals, as a rule, are 
adult males, which appear from five to twelve days in advance of the 
females and immature birds. 
