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Feeding styles at Roebuck Bay


 
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Dr Danny Rogers looks at the wide range of feeding styles that the shorebirds employ on the mudflats of Roebuck Bay. Roebuck Bay, in Western Australia, is one of the most important feeding grounds for migratory shorebirds. Given the huge numbers of birds that this ecosystem supports, it is hardly surprising that there are different feeding styles on view.     
The Terek Sandpiper and the plovers use a search and destroy strategy. These birds scan the surface of the mudflats, run after the prey and grab it.
The Whimbrel employs a similar strategy but specialises on large sentinel crabs which don't venture far from their burrows. While the Whimbrels try to catch the crabs before they go to ground,  they increase their chances of a catch by thrusting their bills into the burrow.
Once caught, the next problem is - how to manage a snappy crab meal.

 
     
Whimbrels will flail a crab from side to side with their beaks in order to weaken and kill their prey. This method of killing allows one of the few competitive birds on the mudflats to battle for the catch. Gull-billed Terns will aggressively harass Whimbrels, in an effort to snatch the catch from their bills.

 
Above: photo by Jan van de Kam and caption from page 108 Life along landís edge. Different shaped bills for different diets! Sentinel crabs burrow deeply underground in an often successful ploy to escape predators like the eastern curlew who has a 20 centrimetre long bill.

Right: photo by Jan van de Kam from page 108 of Life along landís edge. A young common Sandpiper prepares its crab meal by dropping and battering a crab to break off the claws.

 

Red-necked Stints and Curlew Sandpipers are smaller types of shorebirds. They take small worms that are found close to the surface of the mudflats.
It is possible that the the Stints are even able to feed on the microscopic algae that coat the muddy ooze. 

Left: Photo by Jan van de Kam from page 105 of Life along landís edge, shows a Red-necked Stint consuming probably its largest prey - a spider crab. 


 
For food that is deeper down a different feeding style is needed. The Great Knot and its close relative the Red Knot have extremely sensitive receptors at the tips of their bills. These receptors can detect subtle pressure differences beneath the surface of the mudflats. They are looking for the nearest bivalve deep under the surface. To take further advantage, the Great Knot feeds along the tide edge. This is because the larger bivalves, such as clams, start burrowing as soon as the tide ebbs and can burrow safely out of reach. The Great Knot race is a fascinating sight to see, as are the other feeding styles on display at Roebuck Bay.

 
Sequence of photos by Jan van de Kam from pages 106 and 107 of Life along landís edge showing Great Knot hunting shellfish prey by touch.
Left: while the birds forage on both tides, they are far more successful on the ebbing tide, when the bivalves are still feeding near the surface of the mud in the receding waters.

 



Here the Knot, foraging on the ebbing tide, has located a bivalve (Tellina piratica).
The next step is to grasp the shellfish and manoeuvre it out of its muddy burrow.


 


Once extracted, the shellfish must then be swallowed whole Ė shell and all.

 


Once swallowed, the shell will be crushed in the birdís muscular stomach - a sound that can be heard quite clearly if you are close enough to the Knot.

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