320 U. S. NATIONAL MUSEUM BULLETIN 217 



south with numbers permitting on the average 50 percent loss before 

 it returns to the breeding ground in spring. The losses from some 

 populations probably cannot be as much as 30 percent during their 

 absence from the Arctic. These estimates show that a major annual 

 loss from an arctic population cannot be normal and that it must be 

 rare. If an estimated death rate of 50 percent were distributed imi- 

 f ormly over 8 months of absence from the arctic the allowance would 

 be only 6 percent in each month. Some deaths must occur rather regu- 

 larly, because aging is not spasmodic and predators feed throughout 

 the year. There does not appear to be any reproductive provision to 

 offset the possibility that arctic populations may lose heavily at any 

 single stage of migration. 



Energy Resources for Migration 



Hunters and ornithologists well know that storms cause mi- 

 grating birds to fly low and that bad weather may force them to land. 

 After severe weather during the time of migration some birds are 

 found which have perished from exposure or which have been scat- 

 tered to localities so far from their usual ranges that they can never 

 rejoin their populations. Many reports mention birds landing on 

 ships at sea exhausted during stormy weather and it is at these times 

 that birds are most often found injured by collision with lighthouses 

 or buildings. These reports indicate some of the hazards for birds in 

 migration. They do not give any idea of the normal proportion of 

 mischances because in fair weather the flights pass without presenting 

 unusual observations worthy of note. Often the normal flights are not 

 even visible in the darkness or occur on remote migratory routes. 



There are numerous records attesting that birds arrive at the end 

 of long flights over water in exhausted condition. Wetmore (1939, 

 p. 178) vividly described the fatigue and emaciation which he saw 

 prevalent among the small migratory birds landing on the shores of 

 Venezuela from flight over the Caribbean Sea. In later studies at the 

 same landfall Voous (1953) reported that hundreds of blackpoU 

 warblers appeared tame because of fatigue, and remarks that black- 

 polls and redstarts weighed only half what Beebe had reported for 

 these warblers at their assembly points in northern Venezuela, before 

 their return northward. On Dry Tortugas, Florida, Bartsch (1919) 

 remarked that the migratory birds looked worn out in spring and 

 autumn. The condition of the overseas migrants may vary, for an- 

 other observer on Dry Tortugas reported that in autumn the migratory 

 land birds appeared in good shape (Sprunt, 1951) . 



The records of land birds seen over the western Atlantic Ocean 

 during cruises for oceanographic researches have been analyzed by 

 Susan Irving Scholander (1955). Some land birds were found at 

 sea further than 400 miles from nearest land and they must have cov- 



