Natural and Induced Remanent Magnetism in Birds

We measured values for natural remanent magnetization (NRM) and isother- mal-induced remanent magnetization (IRM) in the head and neck for relatively large samples of eight bird species, and smaller samples of 13 additional species. Significant differences were found in mean NRM values among species; values ranged from 3.090 x 10-12 T (tesla) for the Carolina Wren (Thryothorus ludovicianus) to 38.069 x 10-12 T for the Northern Bobwhite (Colinus virginianus). Mean IRM values ranged from 337.6 x 10-12 T in Chimney Swifts (Chaetura pelagica) to 1,889.1 x 10-12 T in European Starlings (Stutnus vulgaris), with intraspecific variation being notably high. For two European Starlings and a Common Grackle (Quiscalus quiscula), about three-fourths of the IRM was located in the head and one-fourth in the neck; heads of two Northern Bobwhites contained an even greater proportion of the IRM. In general, the direction of the ferromagnetic material varied substantially among individuals within species. No significant differences were found in mean-vector directions among species. Linear regressions of NRM and IRM values on the logs of mean body mass indicate that the intensity of magnetism is related to species size. Insectivores, which also were the smallest species, had lower NRM and IRM values than found for omnivores sampled. Characteristic demagnetization-remagnetization curves suggest that most of the magnetic materials are interacting single-domain or pseudosingle-domain grains of magnetite. Three species may contain some superparamagnetic material. No differences were found between migratory and nonmigratory species with respect to the amount of remanent magnetism, or the extent of intraspecific variability in orientation direction of NRM. Received 24 May 1990, accepted 23 September 1991.