I was looking for evidence of culture in animals. Longtime friend and sea turtle biologist, Jack Frazier, suggested I look into oystercatchers. I expected a simple tale of each generation of chicks learning a specific behaviour from its parents.
Eurasian oystercatchers are one of the many wading birds found along the coast of India, northern Africa, the Far East, and Europe. They don’t exclusively eat oysters as their name suggests, but a wide variety of prey from worms and crabs to cockles. The ones that interest me feed on hard-shelled bivalves such as mussels, oysters, and clams.
When molluscs are disturbed, they clamp shut. The more I’ve tried to pry them apart, the tighter the shells stuck together. Despite having primate hands and opposable thumbs, if I’m defeated by these stubborn molluscs, how do birds with nothing more than sharp beaks for tools deal with them?
One group of oystercatchers specialises in stabbing the prey in shallow water. To feed, bivalves have to open their shells underwater. The birds deftly plunge their bills between the shells, and sever the adductor muscle that holds them together. Once the mussels are immobilised, shucking them is relatively easy.
Another group of oystercatchers pries shellfish from rocks exposed by the tide, and batters a hole through the tightly closed shells. Within this specialty, some pound only the anal side, and others, the gill side.
Eating bivalves is a seasonal activity restricted to winters. In summer, oystercatchers poke around in wet soil for soft-bodied delicacies.
Chicks learn one technique of opening hard-shelled molluscs from their parents, and become proficient with experience. When they grow older, they teach their own chicks the same method. We know much of this remarkable behaviour from a paper written in 1967 by Michael Norton-Griffiths, University of Oxford, the U.K.
He swapped the eggs of stabbers and hammerers, and concluded the chicks learn the technique of their foster parents, not biological parents. It was a straightforward case of behaviour transmitted from generation to generation.
If learning one technique takes years of experience, could these birds change techniques later in life? A stabber that suddenly takes up hammering could damage its beak, and the bird might not be able to hunt until the bill grows out. Besides, Martijn van de Pol of Australian National University says an experienced hammerer picks molluscs with shells thin enough to break open without risking beak damage; a beginner may lack this skill.
Stabbing also requires expertise. A mussel can clamp shut around the beak of an inexperienced bird. Since stabbers hunt mussels wedged underground below the waterline, a bird trapped by its prey could be in serious trouble.
On his blog Tetrapod Zoology, British paleontologist Darren Naish republished a photograph of an American oystercatcher, a related species, whose beak had been trapped by a clam buried underground in South Carolina in 1939. The bird drowned when the tide came in.
I’m puzzled there are no recent photographs or reports of inept chicks. If stabbing carries such a high risk, how do babies learn to stab proficiently? Van de Pol explains, “Young oystercatchers stab only small mussels, and there is little risk of getting trapped.” This was the first of the contradictions that plagued me.
Since these birds are such finicky specialists in opening bivalves, I wondered how do they choose their mates? Would a hammerer pair only with a fellow shell-pounder?
Norton-Griffiths says, “I never observed a mixed pair of adults. They always had the same specialisation, both food and technique. Which would suggest that they find mates out on the feeding grounds.”
But van de Pol of Australian National University disagrees. He says in The Netherlands, the birds choose mates with dissimilar specialisations, so females don’t compete with males for food.
Could the English and the Dutch populations have different cultures? Were the English more conservative in their choice of mates? Perhaps.
(To be continued)