The Oystercatcher and the Clam

One of those really cool and useful "evolution stories" gets verified and illuminated by actual research. And blogging!

An oystercatcher is a wading bird of the family Haematopodidae, distributed in one genus, Haematopus. As is the case with many coast loving birds, there has been confusion about the limits of the 11 or so species known to exist worldwide. That itself is an interesting story (Hocke 1996), but one we will not go into now.

Adult coastal oystercatchers (some species are not coastal) eat all sorts of animals found in the intertidal zone, including shellfish of all sorts, depending on availability. They get their name from their tendency to prey on bivalves (including oysters). Oystercatchers have long heavy beaks which allow them to open these bivalves using various methods (de Hoyo, 1996). At least one method they use for this is to jam the beak into the bivalve and cut the muscle that normally would be used by the bivalve to "clam up." This strategy is thought to be dangerous, because if the bivalve closes on the beak, then you've got this damn bivalve attached to your beak for the rest of the day. If the bivalve in question happens to be attached to the substrate (as are oysters and mussels, typically), then the Foraging Fail is more serious; The bird may have shoved its head under water to get at the bivalve. If not, the tide may be on its way in anyway. Either way, the bird may drown.

Here's the interesting evolutionary story, which has to do with development.

Adult oystercatcher photo by Steppeland Click here for attribution

Adult oystercathers have a long beak and long legs to facilitate intertidal feeding behaviors, including wading and bivalve predation. But one method of bivalve predation is potentially deadly. So, baby oystercathers have to learn how to do this. Trial and error is not an option. The very first error a baby oystercatcher makes may be its last earthly act before going off to oystercatcher heaven (and there is no oystercatcher heaven). Instead, they must learn using some other learning method. One might expect oystercatcher genes to be selected to make oystercatchers automatically good at this dangerous act. The problem here is that the neural mechanisms underlying the process are higher order integrative systems using a wide range of sensory inputs and motor commands. Organisms with brains can't evolve pre-programmed genetically tuned neural mechanisms that operate at any level of detail. The brain that runs this finely tuned process must be shaped by experience (learning).

What we see in nature is this: Baby oystercatchers follow their mothers around all day, every day, for many days, watching, watching, constantly watching. They internalize what they are observing, and after many instances of observing the bivalve predation technique, are able to do it.

Baby oystercatcher Photo by Haukur H. Click here for original

That part is interesting; Oystercatchers are an example of evolution NOT solving a complex behavioral problem by pre-programming neural circuits at a fine level of detail. This conforms to what we know about how brains develop (Deacon 1997). It is very difficult or impossible to pre-program complex cortical functions such as language, general intelligence, mathematical abilities, or even something simpler like catching an oyster using genes coding for neural connections. Rather, experience (learning, environment, culture) shapes the brain during development.

And, this part is also interesting (and in relation to oystercatchers, most interesting): The baby oystercatchers, while following around their mothers with their brains being shaped by experience to attain the appropriate skill level, retain a small and ineffective juvenile beak. The babies are incapable of trying what may well be fatal until some time in their development when a hormonal shift occurs, causing their beaks to grow to adult size. It did not have to be this way: It is not true that baby birds typically have non-adult beaks until the last minute (though there is a wide range of developmental trajectories for bird beaks). The long-retained small beak, caused by the timing of hormonal development, facilitates learning in the particular way that conforms to the overall oystercatcher adaptation.

Well, that certainly is a nice story, but it is also based on common knowledge of bird behavior, development, and ecology. How do we really know that oystercatchers actually risk death while foraging for bivalves? Well, we know this because we know this. This is probably one of those pieces of knowledge that is generally known by natural historians, is written down as a generalization in a number of authoritative or semi-authoritative books, and for which there is a handful of anectdotal examples buried somewhere in the pre-PDF, pre-Google, pre-Medline ancient literature. And therefore, lost in obscurity and of no possible value.

But wait, there are scholars who still read actual books and printed journals! And it turns out, this can be useful and interesting. There is indeed an ancient, obscure anecdotal case, and it is brought to us via Tetrapod Zoology Blog. In Clam attacks and kills oystercatcher, Darren Naish describes a publication from 1946 in The Auk (a classic bird journal) in which a brief account is provided of an oystercatcher having got its beak stuck in the clam, as it were.

In this case, an adult Haematopus palliatus (American oystercatcher) got its beak stuck in a Mercenaria mercenaria (hard shelled clam) in South Carolina, in 1939.

It drowned, and the soft tissue of its neck was scavenged by crabs. What a way to go.

Baldwin, W. P. (1946). Clam catches oyster-catcher The Auk, 63, 589-589

Deacon, T. (1997) The Symbolic Species: The Co-Evolution of Language and the Brain. Norton.

Hockey, P (1996) Family Haematopodidae (Oystercatchers) in del Hoyo, J.; Elliot, A. & Sargatal, J. (editors). (1996). Handbook of the Birds of the World. Volume 3: Hoatzin to Auks. Lynx Edicions. ISBN 8487334202

del Hoyo, J., Elliott, A. and Sargatal, J. (1996) Handbook of the Birds of the World. Vol. 3: Hoatzin to Auks. Lynx Edicions, Barcelona.