GULF OF MEXICO 



527 



tidal zone but also occurring elsewhere, we must 

 admit an impossibly large part of the avifauna of 

 the United States, Mexico, and Cuba. Tree 

 growth invades the intertidal zone through the 

 medium of the mangroves, forming the physical 

 basis of a community over the very waters of the 

 Gulf, where a whole host of small land birds must 

 occur as visitants, though their identities remain 

 uncertain since the situation has never been 

 analyzed from that point of view, even in the 

 United States. Even where there are no man- 

 groves, the problem remains. Starlings, as one 

 example, sometimes descend into the intertidal 

 zone in such numbers that they blacken the 

 beaches. But to consider the starling a Gulf 

 bird is to accept a criterion of inclusion that has 

 no practicable limit. Nor can the difficulty be 

 satisfactorily avoided by setting up taxonomic 

 distinctions so as to exclude all species not of the 

 lower orders commonlj^ called water birds. Some 

 passerine birds, such as the seaside sparrow are so 

 strictly Gulf birds that they have developed sub- 

 species virtually endemic to its littoral. Con- 

 versely, some birds that are shorebirds by virtue 

 of their systematic position, such as the upland 

 sandpiper, almost never set foot on earth damp- 

 ened by salt water. 



From the ecological point of view, the birds 

 found on the coast form a continuous gradient 

 from species apparently dependent for their 

 existence on some factor associated with salt 

 water to species that enter the intertidal zone 

 only as intruders. Whether or not they are 

 physiologically capable of doing so, such birds as 

 the brown pelican, the oyster-catcher, Cabot tern, 

 and seaside sparrow seldom venture out of sight 

 of salt water. Some races of the clapper rail, 

 long-billed marsh wren, and sharp-tailed sparrow 

 seem equally bound to salt water; but the species 

 include other races adapted to a life inland. 

 The laughing gull, gull-billed tern, and black 

 skimmer, though mainly birds of the intertidal 

 zone, prove their ability to exist away from it by 

 occasional appearances in the interior. Several 

 kinds of shorebirds, including the red knot, 

 sanderling, and marbled godwit, are almost 

 never found outside the zone of tidewater in the 

 region of the Gulf of Mexico, yet have breeding 

 grounds in interior situations of the northern 

 United States or Canada. The white-faced ibis, 

 the snow and blue geese, the dowitcher, and the 



Wilson phalarope, among others, occur chiefly 

 in the immediate environs of the coast without 

 being notably characteristic of salt water situa- 

 tions. Numerous species, for example, the 

 horned grebe, the greater scaup duck, both yellow- 

 legs, the herring gull, and the least tern, are com- 

 monest on the coast but well represented inland. 

 Contrariwise, the black-crowned night heron, the 

 lesser scaup duck, the American coot, and the 

 spotted sandpiper are well represented on the 

 coast but commonest inland. The birds of the 

 last two categories, together with a host of others 

 that seem to be about equally well represented in 

 both situations, are ecologically linked to aquatic 

 food sources but not especially to salt water 

 sources. It is questionable whether from a 

 strictly objective point of view such species 

 qualify any better as coastal birds than upland 

 species that sometimes derive sustenance from 

 food sources associated with the Gulf. Among 

 such upland species may be mentioned the horned 

 lark and the water pipit, which often feed at the 

 very edge of the surf; the common goldfinch 

 which resorts regularly to the outer beaches 

 attracted by the ripening seeds of the sea oat, 

 Uniola paniculata; and even the peregrine falcon 

 which at seasons shows a preference for a diet 

 consisting of birds of the littoral. Finally, living 

 within sound of the surf, occur a host of birds 

 whose ecological associations are essentially non- 

 littoral. On the narrow wooded ridges of coastal 

 Louisiana, for example, that rise out of the marshes 

 sometimes within a hundred yards of Gulf waters, 

 are to be found breeding such species as the 

 downy woodpecker, crested flycatcher, mocking- 

 bird, white-eyed vireo, orchard oriole, and cardinal. 

 To complicate matters further, the degree of 

 association of various birds with salt water varies 

 in different sections of the Gulf littoral. Along 

 the northern part of the Yucatan Peninsula, where 

 there are practically no inland water features, 

 the water birds represented necessarily occur 

 almost exclusively in the salt lagoons of the 

 coast, even though many of them, including the 

 pied-billed grebe, white ibis, and sora, are else- 

 where primarily inhabitants of fresh water situa- 

 tions. Somewhat similar conditions obtain along 

 the Texas coast where great migratory flocks of 

 water birds are apparently channeled down a 

 narrow flight lane that brings many fresh water 

 species to the very edge of the Gulf. In the great 



