BICKNELL'S THRUSH 213 



have numerous exchange calls, especially when feeding young. They 

 also have a low chook-chook note, invariably used as a barely audible 

 prelude to their song, and also as a scolding note. A rolling, wren- 

 like crr-rr-rr is also occasionally heard. 



A few concluding remarks on the vocal achievements of a young 

 bird kept in captivity for a year may possibly add pertinent data to 

 the moot question concerning the acquisition and inheritance of song 

 and call notes. A caged bird, reared in nearly complete isolation 

 from others of his kind since his fourteenth day, gradually acquired 

 all the call notes characteristic of his race, most of which he had never 

 heard before he gave them. The song, however, actually started 

 when the bird was only 15 days old was always more or less random 

 and experimental. It resembled the song of the adult in tonal quality 

 but was not broken up into the proper sequence of phrases until he 

 was taken back to his native haunts the following summer and given 

 an opportunity to hear the wild birds sing. Even then he often 

 reverted to his off-tune winter song, which had apparently been 

 acquired at least in part by attempted imitation of running water, 

 steam rushing into a radiator, or radio tunes. 



This evidence is in general keeping with the belief that call notes of 

 birds are largely hereditary but that the song is due to both inheritance 

 and learning by imitation. 



Field marks. — Little has been said in preceding paragraphs regard- 

 ing the means of separating Bicknell's thrush from genetically or 

 phenotypically similar forms, which has commonly been considered 

 a problem of some magnitude in both field and laboratory. Over 

 most of its summer range, however, the problem of identification is 

 reduced to a minimum by the absence of all similar forms except the 

 oliveback, which in the New England mountains is the only form 

 likely to be confused with Bicknell's thrush. On Mount Mansfield 

 the albitudinal range of these two birds overlaps slightly at the 3,000- 

 foot level. 



In good view the buffier-toned appearance of the oliveback's head 

 and breast regions is a good field mark, contrasting plainly with the 

 gray cheek of the Bicknell's thrush, and the light eye ring of the olive- 

 back, when visible, is diagnostic. The oliveback's most character- 

 istic call note is a weak, high-pitched pip, quite different from the Bick- 

 nell's sharp, harsh call; and the ascending spirals of the oliveback's 

 song do not closely resemble the Bicknell's ringing, more or less even- 

 pitched refrain. With a little experience, moreover, other minor dif- 

 ferences, hard to describe, but quickly recognizable, are noticeable in 

 their habits and movements. All of these criteria, except song in the 

 fall, can be applied to migrating birds as well as to those in their 

 summer home. 



