Animals rely on an array of environmental triggers or cues to make their behavioural and life-history decisions. In the case of brood parasites, it is adaptive to use a suitable host to guarantee maximum fitness. Despite the fact that the Eurasian reed warbler (Acrocephalus scirpaceus) is the most frequent host of the common cuckoo (Cuculus canorus), we video recorded that cuckoo young often fall out of the nests of this host before fledging time and drown. To establish whether these events are influenced by the size of the nest, we replaced original Eurasian reed warbler nests for much bigger great reed warbler (Acrocephalus arundinaceus) nests and transferred cuckoo chicks into them. Subsequently, we video-recorded the fate of these cuckoo chicks. In these bigger nests, we recorded a significantly lower falling-out rate (4 of 29) compared with those remained in the real reed warbler nests (12 of 32). This result suggests that host nest size plays an important role in the suitability of reed warbler host species and that Eurasian reed warbler nests have a high mortality risk to the relatively big cuckoo chicks. Moreover, we found that cuckoo chicks that fell out of the nests were significantly less fed by foster parents during the three hours before falling-out event than chicks that stayed in the nests. This suggests that also insufficient parental care in terms of lower feeding intensity may facilitate falling-out of cuckoo chicks, probably because hungry chicks are more restless in host nests. The relatively high falling-out rate represents a substantial and underestimated cost of brood parasites. Moreover, without video surveillance it can be incorrectly confused with predation. Therefore, we strongly recommend using video-recording for estimating predation costs in birds.