22 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



petrels of the genus Pterodroma are found. No less than three species 

 of Pterodroma breed (or did) in the Mascarenes: the Trinidad petrel 

 (P. arminjoniana) on Round Island off the north coast of Mauritius; 

 Barau's petrel (P. baraui) and the little known Mascarene petrel 

 (P. aterrima), presumably on Reunion. This latter species is 

 apparently extinct as it has not been seen alive in this century (Bourne, 

 1965) and was not found during an intensive search for petrels on 

 Reunion in 1964 (Jouanin and Gill, MS). Barau's petrel is the most 

 recently described species of petrel (1964), despite its being a fairly 

 common species on Reunion; however, it is not known whether this 

 petrel remains near Reunion throughout the year or disperses to sea in 

 some direction during the nonbreeding season (austral winter). It 

 does appear to confine its pelagic activities during the breeding season 

 to the south and west of Reunion Island, as none were seen to the 

 north or immediately to the east by either Roger Bailey or myself. 

 Since the tropical convergence in the western Indian Ocean is located 

 at approximately the same latitude as the Mascarene Islands (Baker, 

 1965), these observations suggest that Barau's petrel feeds primarily 

 in subtropical waters. Similarly, the location of the only other 

 breeding population of Pterodroma a. arminjoniana (on Trinidad 

 Island in the South Atlantic) at almost precisely the same latitude 

 as the Mascarene population might imply a dependence on the 

 proximity of subtropical waters, but unfortunately this species 

 rarely has been noted at sea. 



Oceanographically, the position of the subtropical convergence, 

 where cooler subantarctic waters sink below the warm water mass 

 of the central (tropical) ocean, delimits the southern boundary of 

 the Indian Ocean (Sverdrup, Johnson and Fleming, 1942). This is 

 a region rather than a well-marked line and generally is considered 

 to lie at about 40° S. Ornithologically it is of interest because it 

 seems to coincide with the northern limits of the distributions of 

 a variety of procellariiform species. Our northernmost records of 

 certain southern petrels, such as the white-chinned petrel and soft- 

 plumaged petrel, were at 31° S; nevertheless, these species were found 

 primarily south of 34° S as were the majority of others including the 

 albatrosses. On a transect in early July, comparable to our 55° E 

 transect, Rand (1962) also encountered the first southern petrels 

 at about 30° S and noted a marked increase just below 35° S; how- 

 ever, the numbers of individuals and the variety of species he recorded 

 were slightly greater than ours and, in addition, he found them as 

 far north as 26° S at stations closer to the African coast. The surface 

 water temperatures recorded by Rand averaged several degrees 

 lower than our measurements. 



