BOBIN 609 



the presence of a pocket-gopher by seeing a plant shake violently when 

 the animal is gnawing at the root. The periods of quiet between the 

 changes of position by the robin probably subserve another function, 

 namely, that of permitting the worms to become active again after their 

 initial alarm. Any person who has dug angleworms for fishing will recall 

 how sensitive the worms are to ground vibrations, even of a very slight 

 nature, and how the worms will withdraw into their burrows and remain 

 there until the disturbance has ceased. It may well be that these quiet 

 poses by the robin give the worms time for reassurance, time to become 

 active again, and thereby to betray their locations to the foraging bird. 

 The robin never digs with its bill as does a thrasher, nor does it scratch 

 out food with its claws as does a fox sparrow. Structurally it is not 

 adapted for either of these methods of foraging, for it is a 'soft-billed' 

 bird, the sign of an eater of insects, worms, and berries. 



In winter, when seeking chiefly vegetable food, the behavior of the 

 robins is quite different. Then, caution as regards noise or movement 

 seems to be almost entirely lacking, and the birds jump and flutter about 

 actively, meanwhile uttering many short notes. They seem excitable at 

 this season, at least when busily foraging, and are often ill content to 

 stay long in one place. But when gorged with food they are likely to 

 remain quietly perched in some leafless tree until the process of digestion 

 has so far advanced that they can begin feeding again. In this latter 

 respect their behavior recalls that of waxwings. 



The song of the Western Robin is a conspicuous feature in the daily 

 chorus of mountain bird voices. It is usually the first real song of the 

 morning. Even before the coming of the faintest streaks of dawn the 

 robins have begun their caroling, and in places where they are at all 

 numerous, as in the Yosemite Valley, the forest and the granite caiion 

 walls resound with their voices. For song perches, they seek the tips of 

 exposed branches such as are reached by the first direct rays of the morning 

 sun. The song consists of a long series of full rounded reverberant notes, 

 grouped into bars with definite rests between. The notes are pretty much 

 all on the same key, yet there is a distinct and alternate rising and falling 

 of inflection. The songs of the Western Robin and Black-headed Grosbeak 

 are sometimes confounded, but that of the latter bird is more varied, is 

 given in quicker time, and it contains many little trills or shrill warbles 

 not to be heard at all in the robin's song. To our ears the song of the 

 robin does not compare in quality with that of many other birds of the 

 mountains, as, for example, that of the Sierra Hermit Thrush and the 

 TowTisend Solitaire. After a time the robin's song becomes actually 

 wearisome because of its monotony. 



