In eastern Australia, two forms of the Australian magpie occur: a white-backed form and a black-backed form. These two forms hybridise across northern Victoria and into South Australia. In this study the response of territorial magpies to caged intruders was examined. Pairs of adult male magpies were introduced into territories. Both were adult black-backed birds, but in each case one of them had its back painted white. The pair was introduced to each territory twice, with the bird that was painted white differing between times. The experiment was run in a population of black-backed birds and a population in the hybrid zone containing white-backed, black-backed and hybrid birds. In both cases, the residents were more aggressive towards the intruder with the white-back than they were to the black-backed intruder. We suggest that this may be because a white-backed bird posed more of a threat to residents than a black-backed bird.