Interactions in the flexible orientation system of a migratory bird

Abstract

MIGRATING birds rely on interacting compass senses: magnetic, star, polarized light and perhaps Sun compasses^1,2. During the development of orientation mechanisms, celestial rotation of stars at night³ and of polarized skylight patterns during the day time⁴ provide information about true compass directions that calibrates the direction of migration selected using the magnetic compass^3–11. It might often be advantageous to adjust the magnetic preference by a geographic reference, especially at high northern latitudes where magnetic declination is large. Paradoxically, a magnetic preference so calibrated will be reliable only within a region of similar declination unless magnetic orientation remains open to calibration in older birds, something that earlier studies suggested was not the case^1,2. We report here that in the Savannah sparrow (Passerculus sandwichensis) the same sort of calibration of magnetic orientation found in very young birds also occurs in older individuals exposed during the migration period to clear day and night skies within a shifted magnetic field.