608 ANIMAL LIFE IN THE YOSEMITE 



(See fig. 60.) At the post-juvenal molt, in August and early September, 

 the young assume a plumage much like that of their parents; birds of 

 different ages cannot thereafter be distinguished readily in the field. The 

 molt of the adults is not completed until about the first of October. The 

 new feathers of the back have an olive tinge, and those of the lower surface 

 are tipped with white; but these markings are lost by wear as the winter 

 progresses, and give way to the clear slate back and red breast. 



The niche occupied by the robin changes with the season. In nesting 

 time the birds live near or upon the ground, save that the males perch 

 high for singing in the morning and evening. They keep to the vicinity 

 of openings in the forest, where there are small trees in which to place 

 their nests and where there is, at the same time, grassland adjacent in 

 which they may forage for worms and insects. The rest of the year they 

 hunt their provender, then largely of a vegetable nature, in trees and 

 bushes, and, for the most part, they fly and perch high above the ground. 



The demeanor of a foraging robin depends upon the sort of food the 

 bird is seeking. When hunting insects or worms on the ground, as in a 

 meadow, it will run or hop several paces rapidly and directly, and then 

 stop abruptly ('freeze') in an erect posture and remain very quiet for 

 several seconds before making another advance. Now and then, a robin 

 so engaged will be seen, at the end of an advance, to thrust its bill down 

 into the turf and get something which is swallowed in a demonstrative 

 manner. Presumably hearing as well as sight plays a part in this kind 

 of foraging, and the short periods of quiet and immobility may be for the 

 purpose both of listening and watching for prey which is moving. 



The ground foraging habits of the robin are quite distinctive, and, to 

 our way of thinking, effective. After a brief but intent survey of its 

 immediate surroundings, if nothing be discovered, the bird moves speedily 

 on to another location, a few paces distant, and there begins anew its 

 close scrutiny. Only a minimum of time is used in changing position. 

 In this way the Robin covers a large amount of territory rapidly, yet with 

 a high degree of thoroughness. It thus combines the habits of two groups 

 of birds which forage in entirely different ways, those which wait passively 

 in one place and watch for moving prey, and those which hunt actively 

 after food which is stationary. The erect posture assumed by the robin 

 obviously gives the bird a wider field of vision from any one spot, and 

 enables it to see better down into the bases of- the grass clumps. The 

 worms and insect larvae which form the bulk of the food obtained in this 

 manner of foraging live, not on the exposed parts of the meadow vegeta- 

 tion, but about the bases of the grass tussocks. The bird probably locates 

 many of these worms by seeing the upper parts of the grass blades move as 

 the M'orms crawl among or feed upon the roots, just as we sometimes detect 



