PELIe'AIs^ ISLxVXD 193 



usual colony liad taken possession of tlie island." 

 (Auk, xiv, 1897, p. 284:,) 



The migration of the island-nesting Terns in the 

 tropics is apparently no less regular. Scott states 

 that the Noddy arrived in the Tortugas " on April 

 20th in large numbers, but remained only two days ; 

 after inspecting their breeding grounds, all departed 

 to return about a week later in greatly increased 

 numbers, when breeding was almost at once com- 

 menced." (Auk, vii, 1890, p. 300.) 



These insular colonies, however, not only throw 

 nrucli light on certain existing phases of bird migra- 

 tion, but they also furnish us with a clew to the 

 origin of migration itself. This is especially true of 

 those species whose lives are passed in the tropics 

 or subtropics, and which we are accustomed tri class 

 as nonmigratory or as "permanent residents,'' but 

 which are as regulaidy migratory, in the real mean- 

 ing of the word, as if they summered within the 

 arctic circle and wintered south of the equator. 



Their movements are apparently in no way in- 

 fluenced by climate nor, at this season, are they 

 governed by the food supply, but prompted solely 

 by the annually recurring physiological change 

 which fits both sexes for reproduction, they repair 

 to a certain islet, perhaps in the heart of their range, 

 with the one object of finding a suitable nesting site 

 in which their eggs may be laid and young reared in 

 safety ; and this object accomplished, thej^ desert the 

 locality, where they may be unknown until the fol- 

 lowing spring. 



Divested, therefore, of the complications which 



ensue when in studying the migration of birds the 

 14 



