Multimodal sensory manipulation of first-year migrating passerines

How navigational cues are integrated during migration is still poorly understood, particularly in birds during their first migration. To test the hypothesis that first-year avian navigation is informed by multiple sensory systems (geomagnetic map information from magnetic particles and olfactory cues), we experimentally manipulated common redstarts Phoenicurus phoenicurus and blackcaps Sylvia atricapilla, undertaking their first autumn migration. We administered either a high-intensity magnetic pulse, or zinc sulphate anosmic treatment, or both. Migratory departure directions were determined in the natural environment using automated radiotelemetry. We predicted that if birds used these sensory systems with redundancy or in a hierarchical manner, single treatments would not change their migratory behaviour, however if birds simultaneously rely on multimodal sensory cues for navigation, the disruption of both senses will lead to incorrect or altered migratory orientation. The sample of birds which received the magnetic pulse only treatment departed in a seasonally appropriate direction equivalent to controls. This standard orientation was however significantly higher in concentration than the anosmic sample, suggesting anosmia disrupts normal migratory directed flight (and possibly vector navigation) in free-flying passerines on their first migration. The response to anosmia is surprising as olfaction has so far only been found to be involved in navigation of experienced birds during subsequent migrations. Furthermore, the sample treated with both magnetic and olfactory treatment oriented differently to other groups. Despite the low sample sizes, this study highlights that landscape level migratory behaviours and strategies are likely fundamental to the interpretation of studies on cue use in migratory animals.