472 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1911. 



It is the homing instinct of birds that renders the Tortugas the 

 most interesting group of all the Florida Keys, for here no less than 

 four species of terns perform their housekeeping, and two of these, 

 namely, the sooty and noddy tern, are not known to breed in any 

 other part of the United States. 



Most of our birds leave their place of birth as soon as they have 

 attained sufficient strength to roam. Many of them perform re- 

 markable journeys in their annual migration from the breeding 

 grounds to their winter home in the fall; and when the breeding 

 season approaches and the reproductive instinct asserts itself they go 

 back to the breeding grounds in the spring. 



The late Prof. W. W. Cooke, has shown that the golden plover and 

 the arctic tern dwell alternately in the Arctic and in the Antarctic, 

 performing a journey of over 11,000 miles twice a year. He has 

 pointed out that the golden plover in one flight covers a distance of 

 2,400 miles without a stop. 



Unlike man, who seems ever ready to shift his tent to where he 

 is afforded the most favorable conditions for existence, most birds 

 cling tenaciously to the immediate surroundings in which they were 

 cradled when it comes to a selection of a nesting site. 



This fact was first demonstrated by sea birds known to breed on 

 certain islands, and on these only. More recently it has also been 

 shown that many of the lesser birds cling equally persistently to 

 their nesting site and it has been found that some not only seek the 

 same general region, but the same shelf of rocks and even the same 

 nest year after year. 



Look at a good hydrographic chart (pi. 1) and you will note 

 that the Tortugas, though situated on the shallow continental shelf, 

 are on the extreme outer limit thereof, away out in the Gulf of 

 Mexico, removed from the murky waters of the southward drift that 

 constitutes the coastal waters of the peninsula, and in a little less de- 

 gree that bathing the upper keys. Here we have the clear water of 

 the Gulf Stream and the first clear water shallows available for a 

 spawning ground to the fishes of the Gulf. The presence of an 

 enormous number of small fish fry at the proper season was, prob- 

 ably more than any other factor, the determining cause in the selec- 

 tion of this site for the rookery by the ancestors of the enormous tern 

 colonies that breed here. It is also quite possible that the factor de- 

 termining the time of arrival and departure of the birds may depend 

 upon the migration and spawning season of the fish used by these 

 birds as food. 



When not on the breeding grounds, the noddy and sooty terns 

 roam in small groups over the waters to the south of our islands. I 

 have met them on both coasts of Cuba and Haiti, where they can be 



