GULF OF MEXICO 



521 



simply "offshore birds" and defined as those 

 species that only rarely come close enough inshore 

 for an observer on the mainland to see them. 



So varied are the relationships of birds with 

 their environment that even this simple definition 

 is not without difficulties in practice. To begin 

 with, it sets up a criterion that is fundamentally 

 distributional rather than ecologic. The brown 

 pelican, for example, often flies out to fish the 

 same waters as the gannet. It spends a great 

 deal of its time, however, resting along the beaches 

 or in the quiet water of the bays. And so we 

 have placed it with the coastal birds. The gannet, 

 on the other hand, while it is linked to much the 

 same food chain as the pelican, almost never 

 comes in to the land or to the harbors. And so 

 we have considered it an offshore bird. Moreover, 

 so little is known about the real distribution of 

 sea birds over the Gulf that in many cases their 

 very presence is merely inferred from their oc- 

 currence in coastal situations after storms. In 

 instances of this sort, the species can be recog- 

 nized as pelagic only by reference to its habits in 

 other oceans. On this basis, the scoters do not, 

 perhaps, strictly fit the definition of an offshore 

 bird, since farther north on the Atlantic seaboard, 

 they are regularly observed from land. But in 

 the Gulf, where there are long shallows, we are 

 inclined to regard the occasional appearance of a 

 few birds inshore as a probable indication of their 

 much commoner occurrence on the deeper water 

 out beyond the range of vision. The old-squaw, 

 as another example, is usually thought of as a sea 

 duck in the same sense that the scoters are; yet 

 in the southern States there have been more 

 records of the species inland than on the Gulf. 

 Consequently, we have felt that the old-squaw 

 does not qualify as an offshore Gulf bird. As 

 these illustrations serve to show, some of the 

 groupings of birds that follow are based on an 

 interpretation of meager facts, are necessarily 

 somewhat arbitrary, and are to be considered 

 merely provisional. 



Even when one employs this rather liberal 

 interpretation of the term offshore, the list of Gulf 

 birds that can be included in that category is a 

 modest one. It amounts to but 24 species, and on 

 the basis of current evidence, only about one-half 

 of them can safely be presumed to occur regularly 

 every year. Not a single one of these species has 

 yet been formally recorded on the Gulf in every 



month of the year, although reports of the white- 

 bellied booby are lacking only in November. Of 

 the other 11 kinds of birds that probably can be 

 found there annually, 7 appear on the face of the 

 record to be winter visitants and 4 to be summer 

 visitants. The seasons of the summer and winter 

 birds overlap in both spring and fall, and it is 

 during these periods, when the domain of the sea 

 birds is invaded by migrant birds of the land and 

 the littoral, that the true pelagic avifauna reaches 

 its own peak of variety and abundance. Only 

 three pelagic species, the blue-faced booby and 

 the sooty and noddy terns, are known to nest at 

 present within the confines of the Gulf. 



Ornithologically, as well as by other standards, 

 the Gulf of Mexico is a tropical sea. Its breed- 

 ing pelagic avifauna consists entirely of species 

 that reach the northern limits of their normal 

 range over its waters, and its scattered offshore 

 islands ' all lie in or near what has been termed 

 the Lower Arid Tropical Life-zone. Thus the 

 Gulf may be expected to share a characteristic 

 feature of warm oceans by presenting vast ex- 

 panses of surface that are virtually without birds. 

 This condition results in part from a well-known 

 principle. The higher the temperature of the 

 water, the lower is its capacity, per given volume, 

 to hold gases and the lower, therefore, its capacity 

 to support the plankton upon which the food 

 potentialities of the sea ultimately depend. As a 

 result of these and other factors, the plant and 

 animal life of the tropical oceans tends to be dis- 

 tributed in depth instead of concentrated in the 

 upper stratum, and it is only where conditions 

 cause a constant upwelling of water from the 

 lower levels bringing up nutrient salts that surface 

 feeders such as birds find an adequate food supply. 

 Such a vertical mixing of strata is likely to occur 

 where two currents come in contact or in the 

 vicinity of islands. 



While the breeding birds of the Gulf are all 

 tropical, its pelagic avifauna as a whole is derived 

 in about equal proportions from the north and 

 the south. Many sea birds, however, that occur 

 commonly in the upper latitudes of the North 

 Atlantic, such as the fulmar, skua, and most of 

 the Alcidae, seem to reach the southern limit of 

 their ranges off New England or New Jersey, 



1 The term "offshore," as applied to islands in this paper, is used to desig- 

 nate those islands separated in all directions from the mainland by 25 miles 

 or more of open sea. 



