Hypervigilance as an adaptive strategy to multiple risks in a songbird

Understanding host responses to brood parasites and predator threats is crucial to elucidating host-parasite coevolution. We investigated auditory vigilance in incubating female Great Reed Warblers (Acrocephalus arundinaceus) using playback experiments and natural sound observations in two populations with contrasting brood parasitism pressure. Females were exposed to the bubbling call of the parasitic Common Cuckoo (Cuculus canorus), the chattering call of the predatory Eurasian Sparrowhawk (Accipiter nisus), and the advertising call of the harmless European Turtle Dove (Streptopelia turtur). Vigilance behaviors (head turns, head alerts, or nest leaving) did not differ between playback treatments or populations. Similarly, responses to natural calls of both threatening and non-threatening species showed no differences. These findings challenge the “parasite-assessment hypothesis”, suggesting that Great Reed Warblers do not adjust vigilance based on local parasite density. Instead, our results support a novel “generalized acoustic vigilance hypothesis”, proposing that warblers maintain consistently high vigilance as a multi-threat defense strategy. This generalized vigilance may be prioritized over threat-specific responses to help mitigate risks from brood parasites, predators, and other nest intruders. Further research is needed to determine if this adaptive strategy is widespread or unique to certain species.