The behavior Shorebirds on mudflat in along Celebes coast
The penetrability of a beach sediment depends on its water content. This may be 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 only 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 reduce 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 resistence 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 have to spend more time feeding to compensate for the loss, but here are advantages in that feeding birds have their heads down searching for food whereas the pirates generally keeps its head up and serves an early warning of the approach of predators (Barnard and Thompson 1985).