Working for food under conditions of varying motivation in broilers

Broiler chickens, Gallus gallus domesticus, have been selected for rapid weight gain and appear to be continously hungry. If this is so, then energy that broilers invest in obtaining food should be insensitive to their level of feed restriction. We tested this hypothesis by measuring the maximum price (i.e. maximum number of key pecks) broilers with different body weights would pay for a food reward Under conditions of varying feed restriction. Two groups of 20 broilers were fed on 50 or 75% of the amount of feed a broiler would eat when fed ad libitum. Broilers were able to learn an operant task and were willing to work for food. Birds of the 50%-group paid a higher maximum price for a food reward and responded more quickly to food presentation. than the birds of the 75%-group in the first test week and showed a nonsignificant tendency to pay, a higher price and to respond more quickly in the second test week. Different levels of long-term feed restriction had an influence on their body weight and on their motivation to work for food. No short-term effect of changing feed. restriction was found. Birds showed frustration behaviour after the last food reward had been obtained, indicating that they were still hungry. The results indicate that broilers are still sensitive to different levels of feed restriction and that they can balance their investments and profits to a certain extent. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.