182 BULLETIN 121, UNITED STATES NATIONAL MUSEUM. 



Spring. — Dr. Alfred O. Gross (1912) has made a most valuable 

 contribution to the life history of this species, based on an exhaustive 

 study of its breeding habits during two seasons at the Bermudas. He 

 says of the migration : 



The tropic birds migrate from the West Indies and, except for occasional strag- 

 glers, none are to be found in the Bermudas during the vrinter months. This 

 annual migration flight is remarkable when it is considered that the birds must 

 necessarily fly over open water for a distance of more than 600 miles without 

 any landmark to guide them. The first tropic birds appear at the Bermudas 

 during the latter part of February, according to the fishermen and local observers, 

 but the great bulk of them dp not arrive unt^ the first weeks of March. Mr. 

 Mowbray, superintendent of the Bermuda Aquarium,, while making a voyage to 

 Turks Island in 1909 saw on February 9 and 10 several groups of two or three 

 individuals each, which were flying in a direct course for the Bermudas. This 

 agrees with the supposed course of migration of the tropic birds and Illustrates 

 the keen sense of direction and orientation which they must possess. These birds 

 would probably be admirable subjects for experimenting on orientation. 



Nesting. — Of the nesting habits he writes : 



Particular localities, especially on the south shore of the main island, seemed 

 to be preferred by many of the birds. At Elys Harbor and Tuckers Town 

 it was not unusual to find as many as 50-75 pairs nesting within a range of 

 less than 100 yards. The tropic birds are not, however, strictly gregarious, 

 for isolated nests about the islands of the sound were very common. The 

 so-called colonies probably exist because of many choice nesting sites, which 

 chance to be situated in the particular locality, rather than to any gre- 

 garious or social instinct on the part of the birds. 



The nature of the nesting site varies from that of the open places on the 

 shelf-like ledges to that of the inner end of a narrow and circuitous passage, 

 or the recesses of an obscure cave. In the two latter situations the presence 

 of the adult bird may often be ascertained by inserting a long pole into the 

 opening, which usually brings forth a shrill cry in response to the intrusion. 

 At Tuckers Town nests were found in shallow excavations in the side of a 

 high sand dune which ran along the shore. These cavities, which apparently 

 were made by the birds themselves, were in each case at the base of some 

 herbage, which to a certain degree shielded and protected the bird from the 

 intense heat and light of the sun. 



The height of the nest above the water varies greatly ; it ranges from a point 

 Just above the high-water mark to one situated near the top of the highest 

 clifCs, perhaps 75 or 100 feet above the sea. At Elys Harbor some of the nests 

 were so low that during an unusually high tide accompanying a storm, they 

 were overwashed by the waves and filled with heaps of sargassum and other 

 sea weeds. The sargassum is found in many of the lower open nests, where it 

 is deposited by the giant waves during the severe tempests of the winter 

 months. No nesting material is ever collected by the birds, but the single egg 

 is deposited on the bare rocks or else on the mat of sea weeds already present. 



Mr. W. E. D. Scott (1891) describes an interesting cave colony, at 

 Jamaica, as follows: 



The cave where the birds were found had a very small entrance, about large 

 enough for a man to crawl into, in the face of the cliff. This was approached 

 only m the calmest weather, in a boat. The entrance led at once into a spacious 

 chamber of irregular shape. Going directly back from the mouth the cavern 



