Predation hazard during migratory stopover: are light or heavy birds under risk?

On the offshore island Helgoland, passerine birds killed by predators (feral cats Felis catus and raptors, mainly sparrowhawks Accipiter nisus) during Stopover were measured and weighed when found freshly killed and still intact. Supplemented by data of migrating birds ringed on Helgoland and predated on the island later on age and body mass of victims were compared to live birds trapped on Helgoland during ringing operations. In the eleven species considered, most predator kills fell within the lightest 20% of birds measured during ringing, regardless of which type of predator was involved. It seems that the risk of being heavy due to fuel loads with respect to reduced escape performance is overestimated. The higher exposure of light birds due to more intense foraging and displacement to suboptimal habitats is probably of higher biological significance by offering conspicuous prey for predators. The lower risk of heavy birds when prey of different body condition is available for predators has implications for modelling optimal migration behaviour. and predation risk is perhaps not an important factor for migrants when deciding on site use.