migratory shorebirds (mostly shallow probers) 

 from intertidal mud flats in Plymouth, Massachu- 

 setts, and learned that these shorebirds caused a 

 huge decline in the density of invertebrate in- 

 fauna on these mud flats during the 2-month 

 (July-September) migratory period. The probers 

 always tended to select the most abundant of the 

 several prey species that made up their diet. This 

 was true on each mud flat examined in the study 

 even though the prey species which were most 

 common varied from one flat to another. The only 

 species which the shorebirds seemed to avoid 

 was Gemma gemma, a small but relatively thick- 

 shelled clam. They fed heavily on polychaetes, 

 amphipods, other small crustaceans, and insects. 

 The largest invertebrates were generally preyed 

 upon more heavily than the smaller species. The 

 most numerous species of shorebirds on 

 Schneider's experimental flats were the sander- 

 ling, semipalmatcd sandpiper, short-billed 

 dowitcher, and black-bellied plover, all of which 

 occur on flats in North Carolina. 



Shallow-probing and searching shorebirds 

 differ among themselves in feeding strategies and, 

 therefore, in diet. The plovers and smaller sand- 

 pipers feed by sight (Recher 1966) and, accord- 

 ingly, prey upon surface fauna most heavily, in- 

 cluding insects and surface amphipods. Most of 

 the other species in this guild (except the .American 

 oystercatcher and perhaps the red knot) have 

 intricately innervated bills which permit them to 

 feed by touch. Feeding by these species is nor- 

 mally a subsurface process of true probing in the 

 shallow layers of the sediments. These birds are 

 thus more likely to take polycha^e worms and 

 other infaunal invertebrates in their diets. A study 

 of two coexisting sandpipers in Sweden (Bengtson 

 and Svensson 1968) demonstrated large differ- 

 ences in feeding habits and subsequent diets be- 

 tween two very similar shallow-probing shore- 

 birds. Other studies (Wolff 1969) demonstrate 

 how probing shorebirds in the same estuary pos- 

 sess different distributions to match the spatial 

 patterns of their preferred prey. 



Other clear behavioral and ecological differ- 

 ences occur among the shallow-probing and 

 searching shorebirds. For instance, the ruddy 

 turnstonc will frequent hard-substrate out- 

 croppings within the mud flat environment. 

 This bird is commonly found feeding around 

 rocks, pilings, and oyster clumps surrounded l)y 



soft sediments. In Beaufort, North Carolina, the 

 ruddy turnstone has been observed feeding upon 

 the wharf roach, Ligia exotica, found on such 

 hard substrates (Simpson 1940). The clapper rail, 

 like all rails, is a secretive bird which hides high in 

 the salt marsh. Rails are more often heard than 

 seen. In feeding, clapper rails often venture out 

 onto the mud flats at low tide, where they can be 

 seen in North Carolina pursuing and capturing fid- 

 dler crabs (Uca) and other decapod crustaceans. 



Oystercatchers differ substantially from the 

 other shorebirds in this guild in that they feed 

 upon large bivalves, such as clams and oysters. In 

 a sense, an oystercatcher is a heavy-duty version 

 of a probing shorebird, distinguished bv its hca\y 

 bill which permits harvest of the large thick-shelled 

 bivalves, which are sufficiently armored for pro- 

 tection against the probing shorebirds. Oyster- 

 catchers worldwide have the distinction of being 

 able to harvest large bivalve molluscs of the inter- 

 tidal zone. In Europe and on the west coast of 

 North America, they consume large cockles and 

 cockle-like clams from this habitat. The oyster- 

 catcher in North Carolina seems entirely restricted 

 to, and therefore totally dependent upon, foraging 

 in intertidal habitats at low tide. 



Although both dowitchers listed in Table 4 

 possess long bills, they appear in the shallow- 

 probing and searching guild because of their 

 feeding habits. Only occasionally do dowitchers 

 probe to the 8- to 10-cm depth made possible by 

 their long bills. Dowitchers frequently use their 

 bills to capture tube-building polychaetes like 

 Amphitrite. By taking such polychaetes, their diets 

 more closely resemble that of the shallow probers 

 than that of the deep-probing shorebirds. This 

 distinction is subtle and the dowitchers could 

 easily be included in the deep-probing guild. 



Most of the species in this shallow prober 

 guild are only abundant in North Carolina during 

 migrations. Some, like the clapper rail and Ameri- 

 can oystercatcher, are year-roimd residents, 

 whereas other species spend either the winter 

 (dunlin) or the summer (Wilson's plover) on 

 North Carolina's tidal flats. Nonetheless, this is 

 largely a migratory guild with high abundances in 

 spring (March-May) and fall (July-November). 

 The spring and fall migrations differ in length and 

 intensity as well as in the direction of net move- 

 ment of the birds. The lall migration tends to be 



54 



