A refined magnetic pulse treatment method for magnetic navigation experiments with adequate sham control: a case study on free-flying songbirds

Migratory songbirds may navigate by extracting positional information from
the geomagnetic field, potentially with a magnetic-particle-based receptor.
Previous studies assessed this hypothesis experimentally by exposing birds
to a strong but brief magnetic pulse aimed at remagnetizing the particles
and evoking an altered behaviour. Critically, such studies were not ideally
designed because they lacked an adequate sham treatment controlling for
the induced electric field that is fundamentally associated with a magnetic
pulse. Consequently, we designed a sham-controlled magnetic-pulse experiment, with sham and treatment pulse producing a similar induced electric
field, while limiting the sham magnetic field to a value that is deemed insufficient
to remagnetize particles. We tested this novel approach by pulsing more
than 250 wild, migrating European robins (Erithacus rubecula) during two
autumn seasons. After pulsing them, five traits of free-flight migratory behaviour
were observed, but no effect of the pulse could be found. Notably, one of
the traits, the migratory motivation of adults, was significantly affected in only
one of the two study years. Considering the problem of reproducing
experiments with wild animals, we recommend amulti-year approach encompassing large sample size, blinded design and built-in sham control to obtain future insights into the role of magnetic-particle-based magnetoreception in bird navigation.