14 
A slightly smaller number of species for the spring of 1884 give exactly 
the same average speed over an average distance of 861 miles. Hence 
it is probable that future observations will not materially change this 
estimate. 
A study of the records for 1883 led to the statement that in spring 
birds migrate more rapidly in the northern portion of their routes of 
travel than in the southern. As this statement was based on the notes 
of one year only, it became a matter of much interest to ascertain 
whether the facts observed would hold good in future seasons and thus 
admit of formulation as a general law, or whether they had been the 
results of specially favorable conditions in the latter part of a single 
season. Accordingly, in the spring of 1884, twenty-five species of well- 
known birds, concerning which we had full records, were selected for 
careful study. The result bears out the foregoing statement. The dis- 
tance traveled was divided as nearly as possible into two equal por- 
tions and the speed was calculated for each. Some of the records do 
not admit of division ; others show an, equal speed throughout; while 
six show an increase of 77 per cent. in speed for the northern half, and 
three show a decrease of 47 per cent. Thus it will be seen that the 
record is strongly in favor of the increase. The same result may be 
reached by calculating the average speed of these twenty-five species 
separately for each of the different months in which migration is per- 
formed; the average speed for March is 19 miles, for April 23 miles, 
and for May 26 miles, per day. The record for 1884 also confirms the 
statement that the later a bird migrates the higher average speed it 
will attain. This would naturally be inferred from the preceding re- 
marks. 
These calculations are averages which give the rate of speed at which 
the bird would travel provided it moved regularly each day. But we 
know that many pauses occur, that on many days there is no advance ; 
heuce, on the days of movement the speed must be much higher than 
that given. Thisis clearly seen in the case of. the Purple Martin. From 
latitude 38° 40’ to 46° its average rate is but 13 miles a day; but we 
have good reason to believe that there was a pause from April 3 to 
April 14, and another from April 18 to May 3. Taking out the first of 
these pauses, the rate is raised between latitude 38° 40’ and latitude 
43° 43/ to 35 miles a day, and, not counting the second pause, the rate 
for the rest of the distance is 28 miles. 
We must also take into consideration the fact that in all probability 
the same bird seldom migrates for several nights in succession, but 
stops to rest after a flight of a night or two, so that the birds migrating 
one night are not the same individuals that were moving the night be- 
fore. 
It has been stated above that the average rate for April is greater 
than that for March, and is exceeded by that for May; but it cannot 
be said that the actual number of miles performed in a night’s journey 
