PECKED CHICKENS NOT SINGLED OUT BY ODOUR

Abstract

Chickens have some nasty habits. When cooped up they can begin pecking each other. Poultry farmers would love to know what triggers this antisocial behaviour. Anna-Carin Karlsson and Per Jensen from Linköping University, Sweden, wondered whether the body odour of individual birds may provoke other birds into attacking them, so they approached Matthias Laska, an olfactory physiologist, to find out how realistic this scenario could be. ‘Birds are thought to be microsmatic – to have a poorly developed sense of smell,’ explains Laska, but when he went digging in the literature he quickly found that some birds rely heavily on their sense of smell. Could Karlsson and Jensen be right? Choosing to test out the body odours of red junglefowl, the common ancestor of all domesticated poultry, the team decided to test whether harassed birds were cursed with a signature scent that singles them out for victimisation (p. 1619).But first the team would have to find out whether individual poultry have individual body odours. According to Laska, poultry only have one skin gland, the uropygial – preening – gland, which could produce an individual's odour. Gently collecting droplets of the secretions from the uropygial glands of pecked and unpecked birds Laska decided to test whether mice could distinguish between the odours of individuals.Training the mice to show that they recognised a smell by taking a drink from a water spout, Karlsson offered a mouse the choice between the odour of the bird that it had been trained to recognise and an unfamiliar odour from another bird. If the chickens smelled the same to the mouse, it would be unable to tell them apart and would respond (by taking a sip of water) to the two odours equally, but if the odours were different, it would respond only to the odour that it recognised. The mice had no difficulties distinguishing between the chickens' odours, so individual chickens can be distinguished based on their odours alone. But could the chickens' odours mark them out as victims?The team decided to test the mice's responses to pairs of chicken smells. If the mice were trained to recognise a pecked bird's odour (carrying a victim signature), the mice would be able to distinguish between the two odours, because one carried the signature and the other did not, but if the mice were unable to detect the signature they would behave exactly the same as if they were being asked to discriminate between pairs of pecked birds and pairs of unpecked birds. Unfortunately the mice couldn't tell the difference, they were unable to detect a victim signature odour. ‘They were too good at distinguishing between individuals,’ explains Laska.Maybe the mice were missing something, so Laska and his colleagues decided to look at the chemical components of the birds' odours. Teaming up with chemists Mathias Elgland, Katriann Laur, Timmy Fyrner and Peter Konradsson, they analysed the volatile components of pecked and unpecked birds' body odours and found that they were largely composed of fatty acids ranging in length from 10 to 24 carbons. But no matter which way they analysed the fatty acid ratios, there was no signature that could signal a bird out for victimisation.Although pecked poultry victims are not selected on the basis of their fatty acid smell, it is possible that bullies detect trace materials in pecked birds' odours or even take offence at their feather colour.