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Thursday, April 3, 2014

Digging for treasure

Do you have a guess about what animal dug this pit in the sand on the beach?  (Warning: The answer is below the photograph.)



On 31 March, we watched a Common Raven (Corvus corvax) actively digging for Mole Crabs (Emerita analoga) at low tideand I mean actively
 
The raven used its large bill to fling sand to either side (note the airborne clumps of sand):



Sometimes it would use its open bill to pick up or push aside large volumes of sand:


The depth of the pits was impressive.  After the raven had left the area, we measured several pits.  They were between 8-10 cm (3-4 inches) deep!


Occasionally the raven would pull a Mole Crab out of a pit.  Some of the crabs were manipulated at the surface before being eaten:



Other crabs were swallowed whole (see below).  Recently we had noticed quite a few soft-shelled Mole Crabs, i.e., crabs that have molted.  We wondered if the raven was choosing to swallow the soft-shelled crabs and to pick apart the hard-shelled crabs?



One more image of this handsome bird, with its sand-covered bill hinting at this intriguing feeding behavior.  How do the ravens know how to find the crabs?  Is it a random search, or have they learned to identify some clue at the surface of the sand?


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