Ivory Towers: First catch your oyster . . .

WHY DO oystercatchers commit adultery? Do the sizes of their beaks have anything to do with it? Two recent papers throw considerable light on these previously under-researched topics.

Heg, Ens, Burke, Jenkins and Kruijt report on their work in 'Why does the typically monogamous oystercatcher (Haematopus ostralegus) engage in extra-pair copulations?' in the December 1993 issue of Behaviour. Their observations provide a revealing picture of what oystercatchers get up to when they are not catching oysters.

After noting that both male and female of these birds are not always faithful to their long-term partners, they attempted to determine 'the possible costs and benefits of Extra- Pair Copulations (EPCs)'.

For males, an EPC offers extra reproductive success and the chance of a superior mate for future reproduction. For a female an EPC might bring extra food from courtship feeding, help with caring for her own chicks, and potential genetic improvements in her offspring. She might also be able to avoid hard work by dumping her eggs in another oystercatcher's nest ('conspecific nest parasitism').

The main costs for the male are that the time spent with his bird on the side might cause him to neglect his own offspring. The female, however, risks alienating her old mate and reducing his contribution to child-rearing and nestwork.

Observation of a large number of EPCs and WPCs (Within-Pair Copulations) indicated that only 7 per cent of successful copulations by males and 5.1 per cent by females were EPCs. Fidelity is at its highest in the month before egg-laying, reaching a maximum of one copulation per hour during low tide.

DNA fingerprinting showed that only 1 out of 65 chicks was not fathered by the long-term partner of the female. Old-established pairs had fewer EPCs than newly-weds.

Opportunity seemed not to be an incentive towards an EPC. Indeed, males whose mates were absent were sometimes seen throwing out soliciting females 'even when there was no apparent risk of a penalty by the mate'. When an EPC happens in full view of the deceived spouse (which is in at least 32 per cent of cases), it may result in aggression, though it was impossible to judge whether this was directed against the mate or the usurper.

There is some evidence to suggest that an EPC may be primarily an attempt to change mate. 'One female switched to a new mate after two years of EPCs with this bird.'

Which brings us to the size of the bill. D H Clayton and P Cotgreave report on 'Relationship of bill morphology to grooming behaviour in birds' in Animal Behaviour, January 1994. Dividing grooming into preening (with the bill) and scratching (with the feet), they point out that a long bill can be detrimental to efficient preening. Conjecturing that long-billed birds would preen less and scratch more than their short-billed relations, they observed the grooming behaviour of 22 species in the wild and 36 in captivity, divided their scratching time by total grooming time, and compared it with bill length. (After a mathematical transformation to factor out the effect of body-weight.)

The conclusion was clear: birds with very short bills hardly ever scratch. 'The sole exception involved pelicans' which didn't scratch at all, but they did have webbed feet, and the only other webbed feet in the study belonged to penguins, who are known to control parasites by preening each other. Foot morphology must play a part in scratching behaviour too. They conclude that 'a comparison of long-legged to short legged birds would be of particular interest'.