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
maximize fuelling rates. However, stopovers also pose mortality risks due
to predator presence. We investigated whether auditory cues from two avian
predators with contrasting hunting strategies—the diurnal Eurasian
sparrowhawk (Accipiter nisus), which captures prey in flight, and the
nocturnal tawny owl (Strix aluco), which relies on pouncing
attacks—trigger different anti-predator responses in the nocturnally
migrating European robin (Erithacus rubecula). We found that robins react
to the perceived risk of predation by the nocturnal predator but did not
alter their strategy in response to diurnal predator cues. Robins reduced
foraging activity under perceived predation risk at night and decreased
their fuel deposition rates over the stopover period. 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.