Nocturnal but not diurnal threats shape stopover strategy in a migrating songbird.

Songbird migration involves frequent migratory flights interrupted by several days of stopover to refuel. For first-year migratory birds, this entails stopping in unfamiliar locations to exploit local resources and maximise fuelling rates. However, stopovers also pose mortality risks due to predator presence. We aimed to determine whether auditory cues from avian predators with differing hunting strategies elicit distinct anti-predator responses in European robins (Erithacus rubecula) during autumn stopover. We exposed captive first-year European robins to calls of either a diurnal predator, the Eurasian sparrowhawk (Accipiter nisus), which captures prey in flight, or a nocturnal predator, the tawny owl (Strix aluco), which relies on pouncing attacks. We monitored changes in daily food intake, body condition, activity levels, and timing of nocturnal activity. Robins react to the perceived risk of predation by the nocturnal predator but do not alter their strategy in response to diurnal predator cues. Specifically, exposure to tawny owl calls led to reduced night-time activity, lower food intake, and slower fuel accumulation, resulting in poorer body condition by the end of the experiment. Lower body condition after stopover can result in a slower migration pace and consequently later arrival to wintering areas, potentially affecting individual fitness. This novel study highlights the flexibility of avian migration programs in adapting to perceived predation risks based on predator activity time and hunting modalities, and how these adaptations differentially shape stopover strategies.