Many animal species protect their eggs against predators while others do not. When these species share the same habitat, the latter species may profit by adding their eggs to those of the protecting species. We show that one tiny predatory mite species protects its own eggs only to a limited extent, and instead adds them to those of another predatory mite species that does guard its eggs, resulting in reduced egg predation. This cuckoo behaviour comes with a cost, and therefore only occurs when egg predators are present. Furthermore, the parasites only add eggs to those of the guarding host species, not to those of another mite species that does not guard its eggs. To the best of our knowledge, this is the first study to show that facultative brood parasitism is more effective than brood care, in increasing offspring survival in the presence of egg predators.