GULF OF MEXICO 



535 



qupntly, and thea only on the northern coast, do 

 conditions of food or temperature place absolute 

 physical limitations on the ability of a bird to 

 survive there. This is especially true of the large 

 and non-insectivorous species that make up the 

 bulk of a coastal bird list. Year after year, as a 

 result, Gulf localities go on making reports of 

 birds "earlier than ever before" or "later than 

 ever before," even in the comparatively well- 

 worked sections of the United States. We may 

 expect the present monthly gaps in the record of 

 occurrence to diminish steadily. 



As a further consequence of climate and position, 

 the patterns of seasonal activity on the Gulf differ 

 rather strikingly from the patterns for the United 

 States as a whole. The regional editors of "Audu- 

 bon Field Notes" for the Gulf States often find 

 themselves handling half of their records of north- 

 ward migration during the so-called Winter Season, 

 from December 1 to March 31. In the stated 

 period of spring migration, April 1 to May 30, 

 they find the major nesting events of the year 

 taking place. And by the time they reach the 

 nominal nesting season, June 1 to August 15, they 

 are already concerning themselves with the first 

 effects of southward migration. This last, some- 

 what paradoxical situation stems from the fact 

 that early southward migratory movement results 

 in the North in invisible subtractions from large 

 summer populations, while in the South those 

 same invisible subtractions are translated into 

 additions to the avifauna or accretions to popula- 

 tions that are at very low ebb. 



III. LAND BIRDS OVER THE OPEN GULF 



In the discussions up to this point, we have 

 been concerned with birds that frequent the region 

 of the Gulf of Mexico and its tidal reaches because 

 of the Gulf: species that are attracted by some 

 particular environmental advantage, or set of 

 advantages, that they find there. Now we turn 

 to a group of birds of opposite relationship, those 

 that occur over this same part of the earth's sur- 

 face despite the Gulf. They are the land birds 

 and the inland water birds that from time to time 

 and in varj'ing numbers have been observed out 

 over the open Gulf during the seasons of migration. 



From the strictly ecological point of view, such 

 birds are not birds of the Gulf at all. But, in a 

 broad ornithological sense, their associations with 



the Gulf are of vastly greater importance tiian 

 those of either the pelagic or the coastal birds. 

 Interposed as the Gulf and its environs are 

 between the summer and winter ranges of scores 

 of highly migratory species, they furnish the most 

 favored testing grounds in the world for theories 

 of bird migration, particularly hypotheses regard- 

 ing the effect of water barriers. 



As specifically related to the Gulf of Mexico, 

 there are two such hypotheses. One is that 

 normal bird migration between South and Central 

 America on the one hand and North America on 

 the other takes place around the edges of the 

 Gulf. According to this view, the records of 

 nonpelagic birds over the open Gulf are of the 

 same nature as records of similar species over the 

 North Atlantic — they represent displaced birds. 

 It has been pointed out that such records are few 

 and that, whenever impressive numbers of indi- 

 viduals have been involved, the associated weather 

 conditions have been such that they might have 

 blown or otherwise shifted birds from overland 

 routes. The most spectacular observations of 

 migrants have been along the Texas coast, and 

 most transient spring migrants have been noted 

 more commonly either in Texas or in peninsular 

 Florida than on the north central Gulf where the 

 bulk of the trans-Gulf flights might be expected 

 to land in spring, if they occur. Indirect evidences 

 of trans-Gulf migration, such as comparative 

 arrival dates in the United States, can be explained 

 without assuming that any major migration routes 

 lie across the Gulf. As originally expounded, 

 these views were carefully confined to spring 

 migration. More recently it has been implied 

 that migrants do not customarily cross the Gulf 

 in fall either. 



The opposing hypothesis holds that, while 

 large numbers of birds, particularly those that 

 winter in Mexico, do migrate northward entirely 

 overland, a significant proportion of those return- 

 ing from South and Central America fly directly 

 across the Gulf from Yucatan. It is pointed out 

 that trans-Gulf migrations may be expected to 

 take place on a very broad front with so much 

 resultant dispersal that the number of birds 

 observable at any one location should be small, 

 and that the only concentrative factors operating 

 on the Gulf are those associated with bad weather. 

 In the. light of these considerations, the present 



