yearly schedule of events in the lives of 

 shorebirds that frequent New England tidal 

 flats. From its Arctic breeding range, 

 which extends from Alaska to eastern 

 Canada, the sandpiper migrates thousands 

 of miles to its wintering grounds along 

 the U.S. Gulf coast and the West Indies, 

 south to northern Chile and Paraguay 

 (Palmer 1967). During migrations, the 

 birds stop at various resting and feeding 

 areas along the eastern coast of North 

 America. In Plymouth, Massachusetts, a 

 minor staging area, peak counts of these 

 birds occur in late July and early August 

 with stragglers present until early Octo- 

 ber (Harrington and Morrison 1S79). While 

 at these stopover areas, the birds do 

 little more than rest and eat, accumulat- 

 ing sufficient reserves of subcutaneous 

 fat to fuel what may be a nonstop flight 

 to the wintering areas in South America 

 (McNeil and Burton 1973) where they remain 

 for 6 to 7 months. In April, the birds 

 start on a return migration to their 

 breeding ranges (Palmer 1967), a trip that 

 takes many to their fall stopover areas. 

 Others take an inland route along the 

 Mississippi Valley. The spring migration 

 occupies less time than the fall migration 

 and after arriving on their Arctic breed- 

 ing ranges, they spend about a month pro- 

 ducing young. They then accumulate in 

 large flocks at major staging areas such 

 as James Bay, Ontario, Canada, and Bay of 

 Fundy, first adults and later juveniles. 

 Soon they depart from the northeast coast 

 and repeat this yearly cycle of events. 



Shorebirds feed primarily on inverte- 

 brates (molluscs, crustaceans, polychae- 

 tes) that are captured on beaches and sand 

 and mud flats. Their daily activity pat- 

 terns and specific foraging sites are 

 often dictated by the tides. During the 

 early part of the ebb tide, foraging 

 begins on the beaches and as the tide con- 

 tinues to recede, many species then move 

 to tidal flats (Burger et al. 1977). Con- 

 nors et al. (1981) related these movements 

 to the peak availability of prey items in 

 these two habitats. During high tide, the 

 birds usually rest on adjacent beaches and 

 upland areas (Harrington et al. 1974). 



Although there are a few large sand- 

 pipers, the majority are among the small- 

 est birds to frequent tidal flats. These 

 exquisitely camouflaged shorebirds often 



go unnoticed by even well -trained eyes. 

 They are probers that often feed in small 

 flocks. Many plovers are larger, m.ay 

 assume a more upright posture in alarm, 

 frequently feed solitarily or in loose 

 groups, and are considerably more active 

 than most sandpipers. Only a single spe- 

 cies of oystercatcher is found in New Eng- 

 land. The American oystercatcher is con- 

 spicuous with a long, bright orange bill. 

 As the name implies, these birds feed 

 almost exclusively on large molluscs and 

 are only infrequently seen. 



A tidal flat may be exploited by a 

 large number of shorebirds of many differ- 

 ent species. Their effects may deplete 

 prey populations (Schneider 1978). Since 

 tidal flats appear to be a physically uni- 

 form habitat, severe competition for food 

 between predator species may be expected. 

 How is it that so many seemingly similar 

 bird species can all exploit the inverte- 

 brates of the same tidal flats and con- 

 tinue to coexist? There are several pos- 

 sible explanations. Due to their migra- 

 tory nature, shorebirds may not deplete 

 resources to the critically low levels 

 that would result in severe competition. 

 When resources are severely depleted, 

 however, we must look for alternative 

 explanations. Among these is the possi- 

 bility that a tidal flat may not be as 

 physically uniform an environment as it 

 appears. If the tidal flat actually 

 represents a collection of discrete micro- 

 habitats, then different species may 

 exploit different habitats with the result 

 that competition is reduced. Differences 

 in sediment grain size, patches of algae, 

 depressions, shellfish beds, cobbles and 

 larger rocks create surficial, horizontal 

 discontinuities while segregation by depth 

 of water and sediments of different prey 

 items represents a vertical habitat divery 

 sity. Superimpose on these variables the 

 temporal component of tidal fluctuations 

 and there exists a wide variety of differ- 

 ent habitats within a single tidal flat. 

 If bird species differ in microhabitat 

 preferences, then foraging individuals may 

 be separated in either space or time, 

 reducing direct competition. In addition, 

 morphology (e.g., bill shape and size), 

 feeding tactics, and prey preferences may 

 prevent even those species that forage in 

 the sam.e areas simultaneously fron actu- 

 ally competing for food. 



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