Double Parasitism by Two Cuckoo Gentes in a Daurian Redstart Nest

Brood parasites lay their eggs in the nest of other species, thereby transferring the costs of parental care to their hosts. Occasionally, more than one female lays an egg in the same host nest, a phenomenon known as multiple parasitism. In the common cuckoo Cuculus canorus, multiple parasitism occurs regularly in some host systems with high cuckoo densities, although its frequency varies substantially among host species and populations. Here, we report a case of double parasitism in the nest of a forest-breeding host, the Daurian redstart Phoenicurus auroreus. Two cuckoo gentes occur at our study site: one lays blue eggs resembling those of redstarts ("redstart-cuckoo"), whereas the other lays grey eggs resembling those of white wagtails Motacilla alba ("wagtail-cuckoo"). Of 182 parasitized redstart nests recorded between 2018 and 2024, 180 were by a redstart-cuckoo and only two were by a wagtail-cuckoo. In the focal doubly parasitized nest, first a redstart-cuckoo egg appeared, followed 4 days later by a wagtail-cuckoo egg. Despite parasitism by redstart-cuckoos being much more common, we never found a nest with two redstart-cuckoo eggs. These observations raise the possibility that interactions among cuckoo females, including potential differences in tolerance or spatial overlap between gentes, may influence the occurrence of multiple parasitism. We therefore hypothesize that female cuckoos may be more tolerant towards females of other gentes, or less effective at excluding them, although this interpretation remains speculative.