MIGRATION AMONG RESIDENT BIRDS 89 



the bird is flightless is not an adequate explanation 

 of this curious distribution, since the channel sepa- 

 rating this island from the one on the east, where the 

 bird is common, is practically dry at low tide, twice 

 in the course of every period of twenty-four hours. 

 Food and shelter conditions are identical on all three 

 islands, so that there is no apparent reason for the 

 anomaly. 



The curious hoatzin {Opisthocomus hoatzin) of 

 northern South America is an excellent example of 

 sedentary habit in a species with continental range. 

 The bird is one with very weak powers of flight, so 

 that it progresses mainly by clambering about 

 through the branches of the thickets in which it 

 feeds. It is said in certain instances to move less 

 than a mile during the entire period of its life, merely 

 coming out in the open in morning and evening, and 

 seeking shelter under leafy branches in the heat of 

 the day. 



Irregular or Vagrant Migration 



Irregular movements among birds may begin, as 

 has been indicated, in wandering of adults or young 

 from the vicinity of the nest site, and may vary in 

 extent from a few hundred yards to many miles. 

 During late summer species that have regular bi- 

 annual migrations may wander in search of food, or 

 young birds may stray from their accustomed range. 



