1SS5.J Drew on the Vertical Range of Birds in Colorado. I I 



ON THE VERTICAL RANGE OF BIRDS IN COLO- 

 RADO. 



BY FRANK M. DREW. 



Colorado, lying between 37 and 41 north latitude and 102° 

 and 107 west longitude, is preeminently a mountain state. Of 

 its 104,500 square miles fully one-half is mountainous, the aver- 

 age elevation of the State being 6000 feet, with extremes of 3500 

 and 14,500 feet. Rising slowly from the Missouri River, the tree- 

 less plains, having already reached an altitude of 3500 feet at the 

 eastern border of the State, thence continue to rise more rapidly, 

 but yet gradually, to nearly halfway across the State. There, at 

 an elevation of about 6000 feet, the outlying foothills throw up a 

 dam stretching north and south the full length of the State. Up 

 into these foothills surge the waves of bird migration in spring 

 to about Sooo feet — the altitude of the Great Parks which stretch 

 their huge treeless surfaces atop the hills. And down these hills 

 comes the return tide of birds in fall, a few to linger near the 

 base, but by far the greater part passing on and down to an alti- 

 tude lower than any found within the State. 



Despite its latitude, which causes very hot summers, the aver- 

 age temperature is below that of other States in corresponding 

 parallels. At an elevation of about 7000 feet, an approximate 

 average for the year gives a temperature of -j- 47 F. ; for 

 winter — December, January, February — of -(- 26° ; spring — 

 March, April, May — of -j- 47 ; summer — June, July, August 

 — of -\- 69 ; autumn — September, October, November, — of 

 -\- 46°. Missouri, in nearly the same latitude, has an approxi- 

 mate mean annual temperature of-j- 55 



The average temperature on the higher peaks, reaching up to 

 from 12,000 to 14,000 feet, usually ranges from 20 to 30 lower 

 than these figures, the difference being greatest in summer. Con- 

 tinued observations at several stations give an average of about 

 -j- 4S for the mean annual temperature at 6000 feet, and of -f-38 

 at 10.000 feet elevation. Timber-line, which varies from 11,000 

 feet to 1 2,000 feet, has an average annual temperature — according 

 to Gannet — of -\- 30 . Notwithstanding the heavy and long- 

 continued snows of winter, and the frequent rains in summer. 



