The extraordinary bivalve-opening specialisations that oystercatchers learn from their parents was fascinating but raised numerous questions.
If a Dutch hammerer started a family with a stabber, would their chicks learn to hammer or stab?
Sarah Dit Durrell of Institute of Terrestrial Ecology, the U.K., and her team showed most birds that probe for worms are females, most hammerers are males, while both genders stab. It stands to reason since females have slender bills, while males have shorter, thicker bills. If feeding technique was determined by sex, then obviously, males and females would have dissimilar specialisations as van de Pol suggests.
However, did that mean the behaviour wasn’t learnt, but came automatically depending on the gender of the birds? Or do girl chicks know to follow mamma and the boys to learn from papa? Van de Pol replies, “We don’t know yet.”
There’s more. What technique a bird uses is not dependent on its sex alone, but also its age. Adults eat hard-shelled molluscs, while chicks eat other prey like crabs and worms that don’t require specialised skill. When one-year-old birds start eating bivalves, they stab. At two or three years of age, they learn to hammer.
Since a male oystercatcher would have likely spent a year stabbing before graduating to hammering, he ought to have little difficulty switching to stabbing should the need arise. But a female may have a harder time hammering because her naturally slender bill is not made for a brawny technique.
Norton-Griffiths also suggested the birds’ choice of technique is influenced by the nature of the substrate. If the soil is soft, hammering pushes the clam or mussel deeper into the mud; hammering requires hard surfaces. Then there are the qualities of the prey: If the shells are strong, firmly attached to the rock, and found in large clumps, oystercatchers don’t hammer.
The birds would have to adapt to the situation to survive the winter, using whatever technique would provide access to the tender flesh of shellfish with least damage to their beaks. To possess knowledge of only one method of opening a bivalve would doom the birds.
John Goss-Custard and another British ornithologist, William Sutherland, refuted Norton-Griffiths’ conclusion that oystercatchers strictly use one feeding technique. Although the birds prefer to use one technique predominantly, they also frequently use another method. Of 10 captive birds they observed, two used all three techniques: stabbing, hammering the anal side, and hammering the gill side. If that is the case, surely the birds would have no problems switching foraging styles when the going got tough.
Are stabbing and hammering taught from one generation to another? The more I thought, the more they seemed to be innate behaviours.
After nine months of reading documents, writing to experts, and making little headway, I felt I was using the wrong technique: hammering at the problem seemed to sink it beyond redemption. I took another stab at the conundrum, going over every paper, verifying every fact, and finally, it became clear.
The 1967 paper offered no evidence of culture in oystercatchers; the main thrust of the paper was the role played by substrate, the quality of shells, and mussel beds in the birds’ choice of technique. Yet the author had inserted two paragraphs in the concluding section of the paper that claimed chicks learn a single technique from their parents that they use throughout their lives, and they teach the same method to their chicks in turn. Some biologists have repeated this myth from document to document, displaying behaviour they expected the birds to possess: cultural transmission of knowledge. And that was the source of confusion.
Four criteria determine how the birds open bivalves: age, sex, substrate, and the character of molluscs in the bed. Not culture.
While it was a relief to resolve the confusion, I was sad oystercatchers were no different from most other birds. But I remain respectful of their dexterity in dealing with hard-shelled molluscs.
Click here for Part 1 of the series.