100 Zoology of Colorado 



has the upper parts of one color throughout, and a buffy ring 

 around the eye. The Willow Thrush (H. fuscescens salicicola of 

 Ridgway) is distinguished from the last by the absence of a buffy 

 or white ring around the eye. The legs are flesh colored in life. 

 It is a rare bird in Colorado. 



Reviewing the Colorado Passeres, we find fourteen families 

 which are common to the Eastern Hemisphere, and only six 

 peculiar to America. But those six are the Tyrannidae, Icteridae, 

 Tangaridae, Vireonidae, Compsothlypidae and Mimidae. It will 

 be seen at once how important an element these are in the Ameri- 

 can avifauna. Whence did they come? Surely not from the 

 other side of the ocean, to have died out in their original home. 

 It is practically certain that they are a part of the ancient series 

 of New World birds, having its evolutionary center mainly in 

 South America. Indeed the invasion of North America by 

 several of these groups may not be so very ancient, geologically 

 speaking, else we might expect them to have crossed to Asia in 

 the days when the northern climate was milder and there was 

 continuous land from one continent to the other. In one case 

 this certainly appears to have happened; we may see in the 

 Troglodytidae an equally American group in its origin, which 

 long ago spread to Eurasia, as did the horses and other animals. 

 In the case of the other circumpolar families, the history is not 

 always clear, but in a good many instances we may safely regard 

 them as immigrants to America from Asia; though often that 

 immigration took place long enough ago to have permitted the 

 evolution of very distinct American genera. But of course it did 

 not take place all at once for any of the complex families. The 

 degrees of resemblance to Old World birds, ranging from complete 

 identity to extreme generic differentiation, must depend partly 

 on the plasticity of the several types, but no doubt much more on 

 their length of residence on this continent. We may pause to 

 wonder why such volatile creatures as birds have not spread 

 everywhere, instead of presenting innumerable local races and 

 genera of variously restricted range.* Our Pacific coasts seem 

 well enough suited to the wagtails, which must have wandered to 

 them many hundreds of times more frequently than they have 



♦Study of the bats of the world has also brought out the fact that the specie* are by no 

 means so wide spread as their powers of flight would lead ua to expect. 



