wider beaches the small ghost crabs Dotilla may occur in thousands with densities of over 100/m2 (Mclntyre 1968; Hails and Yaziz 1982). Although some of the larger individuals are coloured light blue with pinkish legs, the majority are sand-coloured. As the tide rises and covers the beach, each ghost crab builds a shelter of wet sand pellets over its back. Air becomes trapped beneath the crab, and as it burrows down so the air pocket is carried down with it. The crabs emerge as the tide falls, and they are followed by a stream of small bubbles.
Isopod crustaceans can be found by careful examination of the sand and the organic material washed ashore onto the upper shore, and wading birds can sometimes be seen feeding on these and other small animals.
Lower down the beach a variety of molluscs occur but are rarely seen because they burrow beneath the surface. Examples of the bivalves are the white and pinkish Tellina, the large Pinna, and the economically important edible cockle Anadara granosa. This bivalve mollusc spawns seasonally, and breeding seems to be triggered by a drop in water salinity at the start of the wet season. Since plankton and algae on mud always seem to be available, the reason for synchronous breeding is probably to ensure maximum fertilization of the eggs, and to reduce the chance of any one larva being eaten by swamping the potential predators (Broom 1982).
Shorebirds
In addition to four species of resident shorebirds, at least 34 migratory species visit Sulawesi's coasts twice each year. They can be seen between February and April and between September and November, on their way to and from their breeding grounds in northeastern and eastern Asia and their wintering grounds, possibly in northwestern Australia (White 1975). One species, the Australian courser Stiltia isabella, migrates from the south between February and April, and returns between September and November (table 2.5; fig. 2.28). These birds would most often be encountered on muddy rather than sandy shores (fig. 2.29).
Figure 2.27. An Ocypode crab, a common member of the beach epifauna.
Very little is known about the movements of these birds within Indonesia and the basic questions posed thirty years ago have barely begun to be answered. That is: What are the normal migration routes? How many birds are there (Coomans de Ruiter 1954)? EoS teams had the opportunity to work with an ornithologist from Interwader, an international shore-bird study programme, during the first part of 1986, and two areas of mudflat were visited: the north of Bone Bay, and the coast north and south of Watampone. The northern site had extensive mangroves but the mud was rather sandy and, therefore, not especially suitable for waders. One exception was the muddy estuary of the Balease River where at least 18 species were seen, four of which constituted about half of the total number of birds seen. The coasts around Watampone were found to have less sand than in the north, and the shorebirds were consequently more common though of fewer species (Uttley 1986).
* Indicates resident species
White and Bruce 1986
Figure 2.28. Some waders that visit Sulawesi shores, a - broad-billed sandpiper Limicola falcinellus; b - grey-tailed tattler Tringa breviceps; c - rufous-necked stint Calidris ruficollis.
After Beadle 1985
Figure 2.29. Areas of mudflats, the habitat most often visited by shorebirds (indicated in black).
After Salm and Halim 1984
From the above it is clear that the physical composition of the sediment influences the numbers of shorebirds feeding upon it, but within a suitable area of mud it is still not known precisely what attracts birds to one part of a beach and not to another. There are clues, however. Small Ocypode crabs, prawns, fish larvae, polychaete worms and small bivalves are among the most important foods for shorebirds and the distribution of these foods between beaches is very uneven. Differences in the fauna in mudflats can really only be determined by direct investigation (Swennen and Marteijn 1985). Where suitable prey is present, density is the most important factor, followed by prey size, prey depth and the penetrability of the substrate (Myers et al. 1980).
Tidal state, wind and disturbance all affect the density and availability of prey, and this is why certain beaches are only used by the waders at certain times (Evans 1976; Grant 1984). Casts of mud thrown up by suspension feeders and swimming movements of small crustaceans are visual clues for the birds, showing them where to feed (Pienkowski 1983), but some birds use tactile rather visual clues and have sensitive beak tips which can sense prey underground. Sandpipers, one group of partially tactile feeders, may avoid sandy mud because the sand grains are very similar in size to the polychaete and oligochaete worms upon which they feed (0.5-1 mm) (Quammen 1982).
The penetrability of a beach sediment depends on its water content (p. 111). This may be the reason that some shorebirds can be seen running along the water's edge on the ebbing tide pushing their bills into the thixotropic (fluid) sand. A careful examination of bill marks made in tidally formed sand ripples by dowitchers, a wading bird similar to godwits, showed that more marks were found on the crests than in the water-logged troughs. Neither the distribution of prey nor sediment grain size showed any difference between crests and troughs, but penetrating the crests required only 50%-70% of the force required to penetrate the troughs. Thus, concentrating effort on the crests reduced energy expenditure. Ripple crests are sites of active sediment transport and the arrangement of the grains is relatively unstable. This larger volume of pore space allows a higher water content and offers less resistance to penetration. Although the differences in water content between crest and trough are small, minor differences in pore volume can produce major changes in the reaction of sand grains to a shearing force (Grant 1984).
Wading birds are often seen in mixed-species flocks which might be thought to be disadvantageous to the individuals by virtue of increased competition. In fact, more often than not, the birds are taking different foods and being together has the advantage that the more birds present the more likely it is that a predator, such as a bird of prey, will be seen. One particular species is usually first to settle on a certain stretch of beach having used visual clues to make its choice. Other species follow when it is clear that food is being found. A few species act as pirates taking food away from the other species. This is disadvantageous in that the birds which lose food have to spend more time feeding to compensate for the loss, but there are advantages in that feeding birds have their heads down searching for food whereas the pirate generally keeps its head up and serves an early warning of the approach of predators (Barnard and Thompson 1985).
In addition to the waders, other common large birds of the coast include the white-bellied sea eagle Haliaeetus leucogaster, the osprey Pandion halietus and Brahminy kite Haliastur indus all of which fish in the shallow waters and scavenge food along undisturbed beaches. There also various storks, herons, egrets and ducks seen around the shore and roosting and nesting in mangrove forest (fig. 2.30). Palopo Bay and the delta of the Cenrana River are good sites for seeing these birds (Uttley 1986). The milky stork Ibis cinereus is of particular interest because until a few years ago it was thought to be quite rare. Large numbers have now been found in Sumatra (Silvius et al. 1985) and they have also been observed, some in breeding plumage, in the Tiworo Straits between Muna Island and the mainland of Southeast Sulawesi (L. Clayton pers. comm.), near Ujung Pandang and in the Cenrana River delta (Uttley 1986).
Figure 2.30. Large birds seen feeding in or near mangrove forests,
