Ring-billed Gull (Larus delawarensis) 2-20-day-old chicks from a population at Rogers City, Michigan, were used in 680 orientation-cage trials. The apparently innate ability of chicks to select a southeasterly heading, the appropriate bearing for reaching their winter range, was compared when trials were conducted under various intensities of magnetic disturbance (0-7K, which equals disturbances of 0-200 gamma). During minor disturbances (0-3K) a statistically significant proportion of the experimental birds selected southeasterly headings, but during higher intensity "storms" (4-7K) there was a breakdown in such preferences and no statistically significant mean direction was present for the data. Evidence to date suggests that the mode of orientation used by young Ring-billed Gulls for selecting the direction of fall migration is altered by flucutations in the earth’s magnetic field.