214 BULLETIN 196, UNITED STATES NATIONAL MUSEUM 



But the separation of the smaller BicknelPs thrush from the larger 

 graycheek is a more serious problem and has rightly caused some New- 

 England field ornithologists, concerned with the now much-empha- 

 sized bird-listing practices, to question the reliability of sight records 

 where the two birds occur together. Aside from probable differences 

 in song (which could sometimes be used in spring), a slight disparity 

 in measurements is the only criterion that can be used to separate these 

 size races in either field or laboratory. Moreover, a female of the 

 larger (gray-cheeked) race may in some cases be smaller than males of 

 the smaller (Bicknell's) race, and there are, of course, no visible sexual 

 differences in the birds. (Wing measurements in the larger form range 

 from 95 to 108 millimeters, in the smaller form they range from 81 to 

 97, though this slight overlapping is apparently ignored in determin- 

 ation of skins where the sex is known.) 



Obviously, then, only the extremes of these size races can be recog- 

 nized by this means in the field. It has been suggested that the inter- 

 mediate-sized oliveback makes a good standard for comparison; that 

 is, in the rare situation where an oliveback chances to be present, a 

 bird of the gray-cheeked group larger than the oliveback would be the 

 larger northern form, and one smaller than the oliveback would be the 

 " bicknellian" form. These comparisons need to be made with dis- 

 cretion, however, as a maximum-sized oliveback is larger than a mini- 

 mum-sized northern graycheek, and, conversely, a minimum-sized 

 oliveback would be smaller than a maximum-sized Bicknell. Only 

 in dealing with average specimens of all three forms, then, would this 

 interspecific size comparison be useful. 



This leaves only vocal dissimilarities to supplement the usually in- 

 conspicuous disparity in size. To my ear the call notes of the two 

 races offer no detectable differences. According to the few observers 

 who are familiar with the songs of both races on their respective breed- 

 ing grounds, however, there is a noticeable difference in song, the gray- 

 cheek apparently lacking the break and rising inflection that is char- 

 acteristic of the Bicknell's song. The limited singing that occurs in 

 migration, however (rarely in fall, more frequently in spring), is often 

 not full and clear, but merely snatches or fragments of the real song, 

 which my ear usually is not able to distinguish with certainty as to 

 race. 



Most field observers are concerned only with separating the above- 

 mentioned size races, but perhaps it should be added, since many have 

 tried to use color in separating the two forms, that use of color is more 

 apt to confuse than clarify the matter. There are two color phases of 

 Bicknell's thrush in New England — a brownish-backed and a grayer- 

 backed form identical in size ; and two similar color phases of the larger 

 forms, the browner phase in the latter case largely restricted to New- 



