Experimental brood parasitism of the magpie (Pica pica)

Egg and chick features were tested on natural magpie (Pica pica) nests. The stimuli best accepted were those of greater size and weight and of a white background and black specks. Stimuli with a shape different from oval and of low weight were the most readily expelled. Relative quantity of eggs in the nests had no strong influence on the expelling response. Chicks of house sparrows, spotless starlings, jackdaws and swallows were readily aceepted and reared by the magpies. Amongst some chicks of other species introduced, a ‘parasitic’ form of behaviour appeardwhich led them to eliminate competitors in the nests.