BIRDS OF COLORADO. 
17 
DATES OF MIGRATION. 
The notes on migration given in the following table are 
designed to show the different time at which the same species 
of birds arrive at different places and altitudes in Colorado as 
compared with the time of their arrival in the same latitude 
farther east and at a lower altitude. St. Louis, Mo., is thirty 
miles farther north than Fort Lyon, Colo., and one hundred 
and twenty miles south of Loveland, Colo. Hence, according 
•to latitude, the birds should arrive in St. Louis about the same 
time as at Fort Lyon. But it is found that in fact they reach 
St. Louis on the average about twelve days before they appear 
at Fort Lyon. The dates of arrival seem to indicate an aver- 
age difference of six days between Fort Lyon and Loveland. 
The distance between these two places is one hundred and fifty 
miles, or an average movement for the birds of twenty-five 
miles per day. This agrees quite clo.sely with the average of 
twenty-eight miles per day that was found to be the usual 
speed of migration in the Mississippi valley. 
The birds arrive at Idaho Springs about twenty-five days 
later than at Loveland, the result of the nearly three thousand 
feet more of altitude at the former place. 
The records that follow for St. Louis, Mo., were taken by 
Mr. O. Widmann during the spring of 1884, and published on 
pages 33-37 of “Bird Migration in Mississippi Valley.” The 
records for Fort Lyon were made by Capt. P. M. Thorne, U. S. 
A., and those at Loveland by Mr. Win. G. Smith. These 
records were made for the Division of Ornithology and Mam- 
malogy of the Department of Agriculture at Washington, and 
the present writer is indebted to the chief of the division. Dr. 
C. Hart Merriam, for copies of these records. The notes from 
Idaho Springs are those taken by Mr. T. M. Trippe, and pub- 
lished by Dr. Coues in “Birds of the Northwest.” 
It is understood, of course, that when dates are given for 
western varieties that do not occur at St. Louis, it is meant that 
the western variety was noted in Colorado and its eastern rep- 
resentative at St. Louis. Thus, merula migratoria propiuqua 
was seen at Fort Lyon, while 7 nerula migratoria was the bird 
seen at St. Louis. 
All the dates given are those on which the first individu- 
als of the species were seen. 
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