Breeding simultaneously with other members of a population can be induced socially or by the seasonality of climatic conditions or availability of resources. Simultaneous breeding has several fitness benefits, including mate choice, food acquisition or protection against nest predators or brood parasites. In this study, we investigate how spring temperatures at a breeding site affect breeding synchrony and brood parasitism in a great reed warbler (Acrocephalus arundinaceus) population heavily parasitised by the common cuckoo (Cuculus canorus). We predict that in colder springs, the hosts will start breeding later and, thus, their nest attempts will be more synchronised, resulting in a lower proportion of parasitised nests than in warmer springs. Our results confirmed that the hosts started to breed significantly later in colder springs and were more synchronised than in warmer springs. Nevertheless, we found no significant effect of host breeding synchrony on the proportion of parasitised nests. This finding challenges the widely accepted scenario that synchronised breeding leads to a lower proportion of parasitised nests in a host population. It is possible that in strongly parasitised host populations, the high parasite density makes the host breeding synchrony less effective in mitigating the negative impact of brood parasitism. Further research is needed to explore the potential influence of parasite density on the effectiveness of host breeding synchrony as an antiparasitic defence.