Scratch and match: pigeons learn matching and oddity with gravel stimuli.

Two groups of 4 pigeons learned either matching-to-sample or oddity-from-sample by digging in white and black gravel for buried grain. Learning occurred as early as Trial 11, and acquisition was accelerated by as much as 100-fold compared with learning in traditional key-peck environments. Control experiments showed that performance was not controlled by cues other than the gravel stimuli and was not due to distributed practice effects of 8 trials per day and longer intertrial intervals.