The number of brood parasite visits affects subsequent aggression towards the parasite dummy in host males, but not in females

Abstract Aggression towards brood parasites is an important component of host frontline defences. Hosts sometimes adjust this behaviour based on their previous experience with the parasites. Here, we explore whether host aggression towards the parasite dummy is affected by (1) the number of parasite visits prior to the dummy experiment and (2) the time elapsed since the last parasite visit (both identified from continuous video-recordings of host nests). As a model species, we used the great reed warbler ( Acrocephalus arundinaceus ), which is a major host of the common cuckoo ( Cuculus canorus ). At our study site, this host shows high aggression towards the cuckoo and experiences high parasitism rates. We found that the hosts increased their aggression towards the parasite dummy with increasing number of previous parasite visits. Moreover, the hosts retained aggression at high levels if the last parasite visit occurred recently. However, these behavioural patterns were or tended to be significant only in host males. Our findings underscore the importance of individual experience in shaping host aggression towards brood parasites, at least in the more aggressive sex. Therefore, we recommend considering the information on prior host‒parasite interactions when planning dummy experiments, because it may contribute to a deeper understanding of the variability in host defences against brood parasitism.