Getting better all the time: improving preference scores reflect increases in the strength of filial imprinting

In the investigation of the neural mechanisms of filial imprinting, neurochemical measures are often correlated with preference score (PS): approach activity to the training stimulus/total approach in a test. In a previous study, domestic chicks, Gallus gallus domesticus, that had a PS under 65% were classed as 'poor learners' and those with a PS greater than 65% were called 'good learners'. We tested the effects of continued imprinting training in chicks from these two categories. After exposure to an imprinting stimulus for 30 min on day 1 after hatching, preferences were tested and then the chicks were exposed to the same stimulus for either 90 min, 3 h or 4 h on the next day, followed by a second preference test. In all these experiments there was a significant improvement in mean PS in the 'poor learners' between the first and second test, such that these chicks acquired a significant mean preference for the training stimulus. There was no such improvement in chicks that did not receive further training on day 2. When absolute approach was analysed, there was no significant difference between 'poor learners' and 'good learners' at the second test, after 4 h of retraining. Overall, mean preference scores increased with length of training. These results suggest that 'poor learners' are better characterized as 'slow learners', and that their initially low PS is not caused by, for example, a lack of motivation to express a preference. Preference scores reflect the strength of learning during imprinting. Copyright 2000 The Association for the Study of Animal Behaviour.