High Migratory Fuel Loads Impair Predator Evasion in Sedge Warblers

During migration, many species of birds rely onstored fat for fuel. The extra mass taken on for migrationentails costs (Witter and Cuthill 1993). Timeand energy must be devoted to foraging to build upfat loads, and increased feeding may increase therisk of being attacked by predators. An additionalcost of increased fuel loads may be higher predationrisk owing to reduced ability to take off, maneuver,and climb. Mass-dependent predation risk has beenthe focus of several recent theoretical studies (Mc-Namara and Houston 1990, Hedenstro