Tuesday, 24 June 2014

Something eats Fiddler Crabs

Just when I was beginning to believe that fiddler crabs were so good at avoiding predators that predators didn't bother with them, I find evidence of predation.  In a 30 m section of creek bank, I found 3 broken off fiddler crab claws and a partially eaten body.

fiddler crab claw

As I had observed that same creek bank at 9 am, I knew that the predation had occurred during the day as it was 3 pm when I observed the claws.  At night the forests are stalked by the Australian water rat (Hydromys chrysogaster), which is a voracious predator of crustaceans and is like a tiny otter rather than a rat.  Their footprints can often be seen in the mud and on the beach.  So what preys on crabs in the middle of the day.  It was not people – no fresh footprints and people did crabs up which makes a mess.  In fact their were no footprints which made me thing that the predator was a bird.  Birds can carry their victims away from the site of capture before processing their prey and swallowing it.  In this case the large claw was removed.

Without footprints the only way to check out suspects was to photograph birds and look for mud on their beaks.  In the patch of mangrove forest where the crabs were taken were kookaburras, black butcher birds and scrub fowl.  On the sand flats next to the mangroves were several more candidates including a beach stone-curlew and various herons.

Beach stone-curlew, white-faced heron and great heron
My bet is that it was the kookaburra that took the crabs.  Scrub fowl scratch for their prey and generally are not present in the mangrove except when they are moving between islands of beach scrub within the mangroves.  Butcher birds generally attack prey by gliding in with a dead straight trajectory so that their approach creates very little apparent movement in the eyes of the prey.  However crabs are fast and they run when other crabs run and the other crabs with different angles of view would seen the butcher bird move.  Herons seem to be interested in fish not crabs.  The beach stone curlew had mud on its beak, but this species is very shy and is a creature of open spaces.  I doubt that it would go into the mangrove to catch crabs.  That leaves the kookaburra.  Kookaburras often sit on a lawn waiting for something to move so I suppose they could do this in the mangroves.  The photo seems to show mud on the kookaburras beak, however they wipe their beaks on branches to clean them so there is only what looks like a film of dry mud on their upper beak.

Esacus magnirostris
Beach stone-curlew - Esacus magnirostris
Kookaburra with mud on its beak a few metres from the crab remains

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