As the first line of defense in avian brood parasitism, nest defense plays an important role in reducing nest parasitism and increasing host fitness. However, systematic studies on its effectiveness (i.e., on whether it successfully prevents cuckoo parasitism) are lacking. Based on 214 video recordings from 10 years of field observations, we evaluated the effectiveness of nest defenses of the Oriental reed warbler (Acrocephalus orientalis) in deterring common cuckoo (Cuculus canorus) parasitism. The results revealed that, under a breeding pair situation (≤ 2 host individuals), nest defenses of the warbler were hardly effective in resisting the egg-laying behavior of the cuckoo, and many of the cuckoos under attack were able to complete the parasitic process without being harmed. However, when hosts were ≥ 3 host individuals, the trigger value was reached, and the warbler was able to effectively thwart the cuckoo's parasitism and cause lethal damage to the cuckoo. This indicated that the host group was effective in resisting the cuckoo, which fully demonstrates the importance of cooperation and socially transmitted defenses among host neighbors in resisting cuckoo parasitism.