Multiple parasitism in an evictor brood parasite: patterns revealed by long-term monitoring, continuous video recording, and genetic analyses

In some populations of the host of brood parasites, more than two parasite eggs may be laid in a single nest. This phenomenon is known as multiple parasitism, representing a cost to both the host and parasite. In this study, we analysed a long-term dataset (2007–2021) focusing on multiple parasitism of the common cuckoo ( Cuculus canorus ) parasitizing the great reed warbler ( Acrocephalus arundinaceus ). The annual parasitism rate was on average 54.3% and varied between 5.8% and 92.2%, depending on the year. Out of 720 parasitized nests, double parasitism was recorded in 172 (23.9%) nests, triple in 51 (7.1%) nests, quadruple in 10 (1.3%) nests, and, exceptionally, in the years of heavy parasitism (about 90%), quintuple parasitism was recorded in three (0.4%) nests. The rate of multiple parasitism ranged from 0 to 63% inter-annually and strongly correlated with the parasitism rate and the total number of parasite eggs found. Furthermore, the number of cuckoo eggs laid per one nest increased with the decreasing daily availability of host nests that were at a suitable breeding stage for parasitism. Both genetic and egg phenotype analyses revealed that no cuckoo female laid more than one egg in the same host nest. Using data on long-term parasite–host interactions and from continuous video recording, as well as progressive methods to assign parasite offspring thus helped us better understand various aspects of multiple parasitism in hosts heavily parasitized by an evictor brood parasite. Significance statement Laying more parasite eggs in one host nest (i.e. multiple parasitism) is common in brood parasites whose nestlings share the nest with nestmates. In the species where the parasite’s nestling kills its nest mates, multiple parasitism should be rare because it is costly for the parasite. However, in host populations with high parasitism rates, multiple parasitism occurs more often than predicted. Using long-term and video-recording data, we quantified multiple parasitism in the common cuckoo across years and host egg-laying sequences. We found that the rate of multiple parasitism is positively related to the parasitism rate and that the lower the number of nests suitable for parasitism, the higher the number of parasite eggs in one nest. Based on genetic and egg phenotype analyses, we also showed that individual parasitic females avoid laying in the nests they had already parasitized.