io8 THE MIGRATIONS OF BIRDS 



many birds came north along the plains and then 

 spread back into the mountains. Certain species, as 

 some of the j uncos, which had wintered in the foot- 

 hill region, in spring moved down toward the level 

 country and then migrated northward toward their 

 summer home. There were evident thus two lines of 

 movement, one that carried incoming migrants up 

 the gulches into the mountains, and one that 

 brought winter residents down the gulches toward 

 the plains. In August in this same region, such 

 species as the Williamson's sapsucker and western 

 wood pewee, which had nested in higher mountains 

 to the west, moved down to lower regions, and at 

 the same time broad-tailed hummingbirds, western 

 chipping sparrows, and mountain bluebirds in- 

 creased in number, apparently from influx from the 

 same higher regions. There was a distinct tendency 

 among young of the mountain-breeding birds to 

 work down the mountain slopes as soon as the 

 breeding season had closed — a movement that be- 

 came intensified with the beginning of migration 

 from the north in September. 



Similar movements among different species of 

 birds have been observed in many localities. Espe- 

 cially noticeable in autumn are the sudden increases 

 among birds in the edges of the foothills when cold 

 spells occur that spread frost or snow in the higher 

 regions. 



