BIRDS OF THE TRES MA.RIAS ISLANDS. 25 



boat with great curiosity. As they were heard every night while we 

 were at sea, it is evident that they were both diurnal and nocturnal 

 in habits. 



They feed well out at sea, and were not found anywhere along shore, 

 except when they came to their roosting place on Isabel Island. There 

 were no signs of their roosting about the Tres Marias, although they 

 may roost on some of the outlying rocky islets. Grayson found 

 them in small numbers farther west, about the Eevillagigedo Islands. 

 During our trip to the Tres Marias many schools of large fish were 

 encountered swimming close to the surface and constantly breaking, 

 often with such force and rapidity that tbe water boiled and foamed 

 over considerable areas. These schools of fish were commonly acconi. 

 pauied by flocks of sooty terns and gannets, which appeared to be 

 animated by the wildest excitement. The terns hovered over the 

 foaming sea, uttering shrill cries and darting down into the water, evi- 

 dently after food; and in the midst of the turmoil the blue-footed gau- 

 nets swam about, beating the water with their wings and adding to the 

 noise made by the terns and leaping fish. While on Maria Madre 

 I saw a flock of terns some distance oft' shore, and taking a canoe, 

 managed to get out to them, and directly in the course of tbe 

 school of fish they were accompanying. Letting the boat drift, I 

 stood up and watched the swarm go by. Thousands of large fish and 

 hundreds of terns and ganuets passed the boat on every side, amid loud 

 cries from the terns, a rushing sound from the fish and ganuets, and a 

 bewildering complexity of motion in sea and air that was intensely 

 exciting. This novel sight was so interesting that I came near losing 

 the chance to secure some of the birds. 



These terns were seen also following schools of porpoises oft 7 shore 

 in the latter case accompanied by the wedge-tailed shearwater. In the 

 passage between Maria Magdaleua and Maria Cleofa a flock of sooty 

 terns was seen soaring in wide circles high overhead and finally start- 

 ing off for their roosting place on Isabel Island. 



The 'variety crissalisj named in manuscript by Professor Baird and 

 published by Mr. Lawrence, was characterized as "having the under 

 tail coverts tinged with ashy, instead of being pure white." A series 

 of 17 specimens from the west coast of Mexico, and from widely scat- 

 tered islands of the Pacific and Indian oceans, agree in having the 

 posterior part of flanks, under wing coverts, and entire crissum dis- 

 tinctly ashy, not a single individual being white on these parts, as is 

 commonly the case with birds from the Atlantic and Gulf coasts of 

 North America. Unfortunately the series of Atlantic birds at hand 

 is very small, but there is little doubt that crissalis is a valid subspe- 

 cies. Birds from the west coast of Mexico, the Galapagos Islands, 

 and Hawaii agree in having an average shorter bill and tail than 

 those from elsewhere. Specimens from the Indian Ocean have even 

 a longer bill and tail than thos* from the Atlantic, but are ashy below. 



