CHAPTER 5. THE BIRDS 



5.1 THE VARIETY OF AVIAN ROLES ON 

 INTERTIDAL FLATS 



Most visitors to an estuary would probably as- 

 sociate intertidal flats with the various species of 

 wading birds and shorebirds which are commonly 

 seen there. Birds are certainly the most conspicu- 

 ous element of almost any intertidal flat, in part 

 because they are large and usually clearly visible. 

 This subjective imprcssit)n that many birds are in- 

 timately associated with intertidal sand and mud 

 flats is supported by evidence. Fishes can always 

 forage subtidally. Wading and sediment-probing 

 shorebirds, however, can gather food only on in- 

 tertidal and very shallow subtidal flats. Deeper 

 subtidal habitats are largely inaccessible, except 

 to some deep-diving ducks. Furthermore, because 

 of their frequently high abundances and their 

 substantial food requirements resulting from high 

 metabolic rates, birds often have a substantial im- 

 pact on the infaunal invertebrates of intertidal 

 habitats (Schneider 1978). 



Table 4 provides a nearly complete list of 

 those species of birds which utilize the intertidal 

 flats of sounds, lagoons, estuaries, and river 

 mouths in North Carolina. This list is subdivided 

 into six ecological catagories, or 'guilds': 

 (1) waders (including herons, egrets, ibises, 

 yellowlegs); (2) shallow-probing and surface- 

 searching shorebirds (sandpipers, plovers, knots, 

 oystercatchers, etc.); (3) deep-probing shorebirds 

 (godwits, willets, curlews); (4) aerial-searching 

 birds (terns, gulls, skimmers, pelicans, kingfishers); 



(5) floating and diving water birds (ducks, grebes, 

 geese, loons, cormorants, and a swan); and 



(6) birds of prey (ospreys, hawks, eagles, owls). 

 Each of these guilds is ordinarily represented by 

 at least one species on intertidal flats around the 

 world. Certain guilds are clearly more diverse than 

 others. For instance, the shallow-probing shore- 

 birds are the most diverse on almost any shoreline 

 including the flats of North Carolina. In contrast, 

 there are few birds of prey that are important in 

 intertidal systems. 



Many (probably most) of these species of 

 birds represent end-points in the consumer food 

 chains of intertidal flats. Certainly adult waders 

 are largely free of prcdation because of their large 



body sizes, although eggs and nestlings are prob- 

 ably preyed upon by gulls, foxes, racoons, and 

 rats. As adults, terns and gulls are not ordinarily 

 preyed upon by higher-order predators. Only the 

 smaller probing shorebirds (sandpipers, etc.) and 

 some of the ducks are likely to have natural ene- 

 mies as adults. Even for these groups, most nat- 

 ural mortality probably occurs as a consequence 

 of starvation or other factors unrelated to preda- 

 tion. Given that many of the species listed in 

 Table 4 are end-points in the food chains of inter- 

 tidal flats in North Carolina, an important pat- 

 tern in the food chains of estuaries is clearly illus- 

 trated. The usual trophic pyramid is inverted; 

 there arc more numerically important consumer 

 species at the top of the food web. This intertidal 

 flat community organization differs radically 

 from the organization of rocky intertidal benthic 

 communities where distinct food webs, each 

 headed by a limited number of 'top carnivores,' 

 are the rule (Paine 1966). One implication of this 

 difference is that any loss of primary production 

 in an estuarine system will have widespread rami- 

 fications on the populations of top predators be- 

 cause they are all dependent upon the same nar- 

 row energy base. Because so many of these top 

 predators are birds, it is the birds that would suf- 

 fer most from such a hypothetical loss in primary 

 production. 



5.2 WADERS 



Wading birds are characteristic components of 

 the intertidal flat habitat. Table 4 lists all of the 

 wading species that one would ever be likely to 

 encounter feeding on the intertidal flats of North 

 Carolina. Of these, three species are by far the 

 most numerous in this habitat: the great egret, 

 the snowy egret, and the Louisiana heron (Soots 

 and Parnell 1975). These arc also the three most 

 abundant species in rookeries along the entire east 

 coast (Custer and Osborn 1977). This corres- 

 pondence underscores the importance of the in- 

 tertidal flat habitat to wading birds. 



The greater yellowlegs is also characteristic 

 of sand and mud flats. The yellowlegs have been 

 included in the wading bird guild because they act 

 like miniature herons or egrets, wading in shallow 

 water while searching for and ultimately catching 



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