Cool critter of the month: The Eurasian oystercatcher (Haematopus ostralegus)

Phylum: Chordata Family: Haematopodidae

Where do they live? These rather enigmatic migratory birds have a large range stretching across Europe, north Africa, and even into parts of Asia.  Most of the birds spend the winter in the warmer parts of this range, primarily in north Africa and southern Europe, moving northwards to breed.  In some areas like the UK, there are resident populations, where the birds stay all year round (though they undergo a ‘mini-migration’, from the southwest in the winter to the north for breeding).  During the breeding season you stand a good chance of spotting the birds on coastal saltmarshes, sandy or shingly beaches, on dunes and even on the shoreline of inland lakes and rivers.  During the winter months, your best bet is to head down to bays and estuaries.  If there are any oystercatchers nearby there’s a good chance you will spot them.  Not only is the oystercatcher among the largest of the waders, but the birds are also incredibly noisy.  Just check out this short video.

Why are they awesome? The Eurasian oystercatcher is one of 12 species of oystercatcher found across the world.  Apparently they were given their name ‘oystercatcher’ back in the early 1840’s when the American oystercatcher ( Haematopus alliatus) was spotted munching on oysters – a mollusc that is not easy to get at.  Here are three open access papers explaining some of the awesome things we have learned about these flying critters:

Egg or mate… egg or mate Back in the early 1990s, Bruno Ens of the State University of Groningen looked at predation on oystercatcher eggs in a saltmarsh.  It seems that that the eggs are more likely to be predated during the egg laying stage than when clutches are complete.  Based on observation, he puts forward a simple hypothesis on why this may be.  When clutches are complete, either one of both parents are almost always present at the nest, but when the clutches are incomplete, the parents move together between the nests on the saltmarsh and the adjacent mudflats.  So why not leave one parent at the nest to guard the eggs whilst the other heads off?  Bruno thinks that the males are following their female to make sure she doesn’t get down and dirty with another male.

Dumping eggs on your nemesis Many of you have probably heard about the cuckoos sneakily laying their eggs in other birds’ nests but did you know that the oystercatcher is also partial to dumping its eggs in someone else’s nest too?  Clive Craik form the Scottish Marine Institute took a closer look at birds nesting on small islands along the west coast of Scotland and found that 13 species of seabirds were quite partial to this sneaky behaviour.  The oystercatcher was among the most common perpetrator, most commonly dropping them in the nests of common gull (Larus canus) .  Even accounting for the sheer volume of common gull nests on these islands, this is pretty strange when you consider that the gulls are quite partial to eating oystercatcher eggs.  How successfully these dumped eggs are hatched and raised is questionable.

Picking your prey Oystercatchers are quite partial to common cockles(Cerastoderrrea edule).  The cockles shell may offer the little mollusc protection against many threats, but not against the oystercatcher which uses one of two techniques to get at the tasty innards.  The first method involves the oystercatcher hammering its bill against the shell until it breaks through.  The second method involves stabbing their bills between the two sides of the cockle’s shell (valves).  In a study by Ian Johnstone from the Royal Society for the Protection of Birds and Ken Norris from the University of Reading which focused on oystercatchers on the Burry Inlet in Wales, 88% of the oystercatchers used the hammering technique.  Regardless of technique used, you would expect oystercatchers to focus their attention on the big (and thus most energetically profitable) cockles.  Strangely for the oystercatchers that chose the hammering technique this wasn’t always the case, and in early winter hammerers consumed more of the smaller cockles than expected.  It’s not 100% clear why they do this, but may be down to the amount of energy an oystercatcher needs to expend to survive winters.  This extra energy expenditure means less energy to break their way into the cockles, so smaller cockles may serve as a happy medium.  Hammerers also avoided eating the really big cockles, possibly to reduce the risk of breaking their bills whilst trying to make a hole.

Image:  This oystercatcher with a 3 day old chick was snapped on my very Island (Jersey) by Deryk Tolman.  Check out some more of Deryk’s work on his Flickr site