Dr Rodenburg and German researcher, Alexandra Harlander-Matauschek, jointly studied the results of using the bitter substance quinine on the feathers of egg laying hens. For 25 weeks, right from when the chicks were at their youngest, Dr Harlander monitored six groups of ten chickens which engaged in a lot of feather pecking, and six times ten chickens which showed little signs of feather pecking. Both groups came from a Danish breeding programme in which ten generations of laying hens were selected based on much and little feather pecking. For both groups of chickens, the quinine spray reduced the incidence of feather pecking by half, reported Drs Harlander and Rodenburg in the November issue of Applied Animal Behaviour Science.

However, the spray has to be used continually to reduce feather pecking.

The researchers said: "The research was an experiment to find out if we can inculcate into chickens the thought that feathers are dirty. Do the chickens retain this thought when the spraying stops? Unfortunately, they don't. You have to keep spraying the bitter stuff to get results."

Yet, poultry farmers can benefit from this research.

The added: "They can spray problematic pairs of laying hens with quinine or some other bitter substance to stop damages caused by feather pecking. If this can lead to less mortality among laying hens, such a treatment would have quickly earned its keep." The German experiment worked with white Leghorns; other chicken breeds are also used in poultry farms. Dr Rodenburg does not know whether the bitter substance also produces such good results in the farms. In Wageningen, his colleague Piter Bijma tries to select hens which lay many eggs and engage in very little feather pecking; this research project is still going on.