The incredible bill of the oystercatcher

Inspired both by the clam catches oystercatcher story, and by Greg Laden's coverage of oystercatcher learning and predation behaviour, I thought it an opportune time to recycle the following from Tet Zoo ver 1. It originally appeared as one of my Ten Bird Meme posts of 2006...


One of my most favourite birds is the extraordinary, charismatic, beautifully interesting oystercatcher (meaing Eurasian oystercatcher Haematopus ostralegus: adjacent photo by Bjørn Christian Tørrissen, from wikipedia). One of ten or eleven extant haematopodid species, it sports pied plumage, pinkish legs, and has the heaviest bill of any extant wader. One of the most interesting things about oystercatchers is the fact that they exhibit resource polymorphism, with some populations exhibiting multiple different forms (Skúlason & Smith 1995). 'Stabbers' feed by jabbing their laterally compressed bill tips in between the valves of a mussel's shell, while 'hammerers' crack open mussel shells by pounding on them. Some hammerers only break in to the shell on its dorsal side, while others only break in to the ventral side. Others attack only the left side valve, and others only the right valve. Other Eurasian oystercatchers are worm specialists with pointed tweezer-like bill tips. Superimposed on this variation is sexual dimorphism: females have longer, heavier bills than males (bill dimorphism of this sort is now known to be present in many birds) [image below, from Hosking & Hale (1983), shows a worm-eating bird on the left and a 'hammerer' on the right. There are other images that better show the variation (there's a more impressive one in Sutherland (1987)), but I only have poor, very dark photocopies of them)].


First discovered by M. Norton-Griffiths during the 1960s (Norton-Griffiths 1967) - and extensively studied by a great many ornithologists since then - resource polymorphism among oystercatchers was initially thought to be learnt by the birds from their parents (and not genetically determined). It now seems that things are far more flexible, with individuals switching from one behaviour to the other over the years. It's been said that juveniles can't really learn how to handle prey from their parents given that many of them are reared inland and are abandoned by their parents before they ever get to the coast (Sutherland 1987). However, some oystercatcher adults that specialise on mussels spend up to 26 weeks teaching their young how to exploit prey (the long apprenticeship of the oystercatcher - longer in those that stab or smash mussels than in those that eat worms or exploit other prey - is well established in the literature: e.g., Wunderle (1991), Safriel et al. (1996)) [in the photo below, by John Haslam, from wikipedia, the adult has provided the juvenile with worms. The pointed tip on the adult's bill shows that it's a worm-catcher].


It seems that it's the behavioural flexibility that controls bill shape, rather than the other way round, and another remarkable thing about oystercatchers is how specialized their bills are for coping with wear. Uniquely among waders, the bill grows at a jaw-dropping 0.4 mm per day (that's three times faster than the growth rate of human fingernails). This rapid growth means that the bill can change shape very rapidly if the feeding style is changed, and captive individuals that were forced to switch from bivalve-feeding to a diet of lugworms changed from having chisel-shaped bills to tweezer-like bills within 10 days. A-maz-ing.

Given that oystercatchers are fairly large and powerful for waders, and able to smash open bivalve shells, it follows that they are formidable and potentially dangerous to other birds. Certainly males will chase off raptors when defending nesting females. I recall reading accounts of them caving in the heads of other waders during territorial disputes, but unfortunately I can't remember where (a common problem, despite my well organized library). Most aggressive interactions recorded between oystercatchers, and between oystercatchers and other waders, involve piracy, and in fact some birds obtain most of their food this way, "attacking other birds at an average of five minute intervals during low tide" (Hammond & Pearson 1994, p. 61). As much as 60% of the food of some individuals is obtained by piracy. Finally, oystercatchers are incredibly long-lived, with the record-holder dying at age 35!* Now, come on, that is a truly extraordinary bird.

* Since I wrote this text, a Eurasian oystercatcher that reached the age of 40 has been reported.

For previous 'Ten Bird Meme' articles on Tet Zoo see...

And for articles on bill morphology and function in birds see...

Refs - -

Hammond, N. & Pearson, B. 1994. Waders. Hamlyn, London.

Hosking, E. & Hale, W. G. 1983. Eric Hosking's Waders. Pelham Books, London.

Norton-Griffiths, M. 1967. Some ecological aspects of the feeding behaviour of the Oystercatcher (Haematopus ostralegus) on the Edible Mussel (Mytilus edulis). Ibis 109, 412-424

Safriel, U. N., Ens, B. J. & Kaiser, A. 1996. Rearing to independence. In Goss-Custard, J. D. (ed) The Oystercatcher: Individuals to Populations (Oxford University Press, Oxford), pp. 210-250.

Skúlason, S. & Smith, T. B. 1995. Resource polymorphisms in vertebrates. Trends in Ecology and Evolution 10, 366-370.

Sutherland, W. J. 1987. Why do animals specialize? Nature 325, 483-484.

Wunderle, J. M. 1991. Age-specific foraging proficiency in birds. In Power, D. M. (ed) Current Ornithology, Volume 8 (Springer), pp. 273-324.

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Another interesting thing about oystercatchers is that two races developer recently. In the Netherlands, population growth overfilled all the shore teritories, forcing some Oystercatchers to nest inland.

These inland oystercatchers fly over the territories of their kin to feed on the low tide. They carry food for their young in flight one by one, to which they are poorly adapted. Also, their feeding territory is exposed by tides for much shorter time. Therefore, their breeding success is much lower. Remarkable is that inland oystercatchers are stuck for life - over decades maybe 1% of them manage to switch territory to the coast, which involves lots of fighting.

Which is remarkable for me how easily different behaviours appeared, and how easy is to mistake them for the genuine cryptic species.

some oystercatcher adults spend up to a year teaching their young how to exploit prey: in fact a photo in Attenborough's The Life of Birds shows an adult opening a shell while a juvenile, at its side, watches with apparent interest

gah. The latter observation is of course in no way evidence for the former assertion.

By Sven DiMilo (not verified) on 11 Jul 2010 #permalink

Incredible. I am now moved to consider trophic polymorphism in relation to primary vs. fallback foods vis-a-vis primate/ape/human evolution. Yet another theory that will explain everything in human evolution!

I went ahead and nominated this post for I and the Bird. I hope that's OK (if you had a different post in mind, Mike B would just swap them, or even better, use them all!)

Sven (comment 2): gah, you are being so pedantic! The reference to a picture in The Life of Birds was not intended as the 'observation' that supports the assertion: the assertion (that juvenile oystercatchers undergo a lengthy apprenticeship) has been established elsewhere in the literature. I've now edited the text.

Heck - they're stunners. If Len Wein had read this blog back in 1974, I reckon Oystercatcher would've ended up in the X-Men, rather than Wolverine.

A better X-Man yet: The Tardigrade -- !! Able to withstand absolute zero and boiling temperatures!! And radiation!! And -- THE SILENT VACUUM OF SPACE ITSELF!!

Actually, that's not a bad idea.

In addition to behavioral and phenotypic plasticity, oystercatchers can also be used as a fine example of evolutionary plasticity (the fashionable buzzword being "evolvability"). Consider that oystercatchers, stilts, avocets, and ibisbills constitute a clade within the morphologically conservative plovers. (You may want to add wrybills and Magellanic plovers to the mix too.)

By John Harshman (not verified) on 12 Jul 2010 #permalink

Here in NZ we get two species (Pied and Variable) of oystercatcher whose ranges overlap. It'd be interesting to know what the feeding differences are between these two superficially similar (except sometimes in plumage) looking spp.

I'm always sceptical they even are different species but haven't done any work to look at why the are considered different admittedly.

By RStretton (not verified) on 12 Jul 2010 #permalink

Yeah, I've often casually watched the NZ variable oystercatchers at work on reefs of mussels - so far I've only seen "stabbers". They seem to target the half-grown mussels about 4cm long. I guess that's the best strategy in terms of effort vs. pay-off; they take only about 30-40 seconds to deal with each one.

Picking your prey in plain sight at high tide would also avoid the problem of picking shellfish that are too big to subdue, as we saw in the previous oystercatcher post...

By ian govey (not verified) on 12 Jul 2010 #permalink

Given their lethal equipment and flexibility, why haven't any oystercatchers turned to predation on other shorebirds? Or have they?

By Nathan Myers (not verified) on 13 Jul 2010 #permalink

I'm not aware of any bird-predation attempts from oystercatchers. I think that the molluscs and annelids they prey on are more than numerous enough, so no pressure to switch to a far more awkward prey item (sure would be interesting if a big-billed shorebird did become a bird predator).

Do skuas count, or aren't they big-billed?

By John Harshman (not verified) on 14 Jul 2010 #permalink

"Now, come on, that is a truly extraordinary bird."

What isn't in your hands? And that's a compliment. I haven't seen anyone do such a good job of bringing out the magnificent in the seemingly mundane.

By Mike from Ottawa (not verified) on 14 Jul 2010 #permalink

I love Oystercatchers - a pair nested in a field behind our house last year (the land is reclaimed tidal marsh). Once the chicks hatched the male was super-aggressive, dive-bombing the heck out of us whenever we went near. No sign of them this year, but we got Lapwings instead, so it's all good...


Another interesting thing about oystercatchers is that two races developer recently. In the Netherlands, population growth overfilled all the shore teritories, forcing some Oystercatchers to nest inland.

Locally in north-western Europe, Eurasian oystercatchers have started breeding on the roofs of buildings in cities. And they have learned to forage on the lawns of city parks, probing for earthworms. So yes, they are quite adaptable.


Given their lethal equipment and flexibility, why haven't any oystercatchers turned to predation on other shorebirds? Or have they?

Yes, they have, actually; the Eurasian oystercatcher Haematopus ostralegus, at least, does eat other birds' (mainly small larids') eggs and young on occasion.

(Incidentally, the ruddy turnstone Arenaria interpres, a much smaller shorebird, is locally a habitual if not a downright notorious egg thief. It breeds preferentially near tern colonies, and whenever the terns leave their nests - for instance to chase away a gull or a crow - the turnstone seizes the opportunity to predate eggs. Curiously, terns do not seem to learn to recognise the turnstone as a threat.)


sure would be interesting if a big-billed shorebird did become a bird predator

Well, thick-knees (or stone curlews, if you prefer) Burhinus regularly prey on small vertebrates such as lizards and rodents, and given the chance they will surely prey on the young of larks and other small ground-nesting birds as well. I wouldn't be surprised to learn that thick-knees are capable of dispatching small adult birds too.

I haven't done much adventuring in my life, but last summer I climbed out on a rocky jetty with crashing waves around me. Less than wise, but for my efforts, a mixed flock of ruddy turnstones and surfbirds was my reward. Very cute, but hard to get close to. I couldn't ID them (no expert) until I got home, because the closest thing I could find to a birding book locally was a tourist guide pimping oystercatchers and avocets - of which I saw not a one.

The idea that cute little turnstones snipe eggs from fierce scary terns is morbidly amusing to me. Thanks for that image, Dartian.

Charadridae rule the world. They were jetting between continents long before we worked that trick out. Can you imagine how cool it would feel for an animal, to have the entire freakin' planet to choose from, for feeding grounds and rookeries? If you're a good human, your next incarnation up is as a seagull.

By CS Shelton (not verified) on 17 Jul 2010 #permalink