OLIVE-BACKED THRUSH 185 



Although rarely heard in the daytime except in cloudy weather, they sang 

 almost continuously through the night. One of the greatest delights of summer 

 camping in the Yukon Valley is to lie in one's blankets at night listening to the 

 ringing chorus of these thrushes. If the camp be fortunately chosen, one hears 

 not one or two but a score of songsters. First we may have a bird less than 20 

 feet from the tent whose every utterance is audible, varying from tones of the 

 greatest depth and richness to exquisite inarticulate gurglings and confidential 

 whisperings. Then a few rods farther away may be several others alternating 

 with one another in a long-continued obligato, while still farther back in some 

 small ravine are those whose songs are borne on the air with a slight reverberation, 

 giving added charm. While we lie in delicious enjoyment of these nearer songs, 

 a general sense of music pervades the air to the farthest echoes. Perhaps there 

 is a momentary lull, a sudden silence crowded with expectation. Then from a 

 deep canyon beyond the wooded ridge behind us comes a far-away note, faint 

 but full of character, and though little more than an echo, still with a tone that 

 thrills. In the same way other notes, or a whole chorus, faint but sweet, are 

 borne from the distant thickets across the river. 



Field marks. — The four common thrushes that migrate through 

 the United States can be readily recognized by the color pattern of 

 the upperparts. The wood thrush is more rufous on the head and 

 more olivaceous toward the tail; the hermit thrush is just the reverse, 

 more olivaceous toward the head and more rufous toward the tail, 

 especially on the tail; the olive-backed and russet-backed thrushes 

 are uniformly brownish olive above; and the upperparts of the veery 

 are uniformly more rufous. Furthermore, the wood thrush is more 

 conspicuously spotted with larger and blacker spots on the breast; 

 and the veery is very faintly spotted there with obscure spots. The 

 gray-cheeked and Bicknell's thrushes, which are rarer, cannot be 

 easily distinguished from the oliveback by the color of the upper- 

 parts; it is necessary to see clearly the buff cheeks and the conspicu- 

 ous buff eye ring of the oliveback to recognize it. 



Enemies. — The nests of the olive-backed thrush are so conspicu- 

 ously placed and usually so accessible that they are very vulnerable to 

 the attacks of crows, jays, squirrels, other wild predators, and stray 

 cats. Mr. DuBois says in his notes: "At nest 5, small pieces of broken 

 egg shell in the nest and a larger piece on the ground indicated destruc- 

 tion by some animal. Nest 6 contained two fresh eggs, one of which 

 had a large hole in the side and had been partly eaten. A chipmunk 

 was chattering and running in the brush not far away. The thrush, 

 nearby, had been calling its alarm note." 



Dr. Friedmann (1929) records it as "an uncommon victim" of the 

 cowbird, five cases having come to his notice. 



Taverner and Swales (1908) say that this and the gray-cheeked 

 thrushes, on migration at Point Pelee, "suffer greatly during the 

 Sharp-shinned Hawk flights as mentioned before. During the periods 



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