BIRDS OF COLORADO. 1 7 



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 closely 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. Wm. 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 propinqiia 

 was seen at Fort Lyon, while merula 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. 



