What makes free-range broiler chickens range? In situ measurement of habitat preference

In commercial free-range broiler (meat chicken) systems, many birds never leave the houses, making them 'free-range' in name only and suggesting that the environment provided is not preferred habitat. We investigated the factors that affect the numbers of birds ranging in 40 flocks, each of 20 000 growing broiler chickens, Gallus gallus domesticus, in 14 houses on seven commercial farms and at three seasons of the year. The number of birds found to be ranging outside was positively correlated with the amount of tree cover the range area contained, the time of day and the season of the year. Few birds were seen ranging in the winter but even in summer, the maximum number observed outside during daylight hours at any one time was less than 15% of the total flock. We measured habitat preference of the chickens, using a statistical method previously used for wild birds, and argue that this 'in situ' measurement of habitat preference could be widely used in a variety of situations and that it thus constitutes an important noninvasive and nonintrusive method for assessing animal welfare on commercial farms. (C) 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.