The oystercatchers

I shall now look at them with new eyes indeed!  This article was linked by Thyagu in our MNS group, as a response to pictures and idying issues.

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…

• Ifrita the poisonous passerine
• Pseudopodoces, the corvid that wasn’t
• B. rex!
• Attack of the flying steamer ducks
• Fish owls in reverse

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

• The godwits’ many bills
• An encounter with a crossbill
• Coccothraustes: most bizarre of finches
• Sexual dimorphism in bird bills: commoner than we’d thought
• Woodpeckers: barbed tentacles and the avoidance of brain injury
• Mysterious channels of Alca torda
• Yes, it was a kiwi
• How to prevent cannibalism in pheasants
• The little-known subgenre of Talpanas tribute art

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). Ibis109, 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.