THEORIES OF MIGRATION 31 



simplest phase, a matter often of yards, though it 

 may extend to greater distances. 



In some forms of bird-life found in tropical regions 

 the succeeding step may be exemplified. Climatic 

 conditions there are quite uniform, divergence from 

 a level mean being found mainly in the amount of 

 precipitation that marks wet and dry seasons. In 

 careful observation over a limited area certain birds 

 may appear extremely rare for the greater part of a 

 year. Suddenly some forest tree will come into 

 flower or fruit, and immediately these same birds 

 flock, at times in abundance, to feed. They remain 

 common for a period and then disappear. These 

 shiftings among tropical forms are little understood. 

 Many ant-thrushes, flycatchers, and other small 

 brush-birds undoubtedly are wholly sedentary from 

 year to year; but many others are certainly erratic 

 in their occurrence. Years ago Dr. C. W. Richmond 

 found the beautiful snow-white cotinga known as the 

 holy-ghost bird of rare occurrence in a certain sec- 

 tion of Nicaragua for a period of several months. 

 Later with a slight change in the season it became 

 almost common. The little euphonias, brilliant gems 

 in the group of tanagers, are governed in their wan- 

 derings by the ripening seeds of mistletoes, which 

 form their sole food. Hummingbirds appear and dis- 

 appear with the flowering of vines, trees, or certain 

 epiphytes, their presence being governed wholly by 

 these phenomena. 



