Natural and Induced Magnetization in the Bobolink, Dolichonyx Oryzivorus (Aves: Icteridae)

Abstract

The magnetic characteristics of the heads from 28 bobolinks [Icteridae: Dolichonyx oryzivorus (L.)] were analysed using remanence magnetometers. The natural remanent magnetization of 12 freshly preserved heads averaged 3.20xl0−7 electromagnetic units (e.m.u.) (l e.m.u. = 103Am−1) and was oriented horizontally from side to side. An electromagnet was used to induce magnetization with external field strengths of up to O.7 Tesla (T). The average saturation isothermal remanent magnetization (sIRM) for 23 bobolink heads was 2.49×10−5e.m.u. Because the IRM was attained partially below 0.1 T and totally below 0.3 T, we believe that the magnetic material in the bobolink is at least partially single domain magnetite. The relationship between the IRM acquisition and alternating field (a.f.) demagnetization curves indicates that the single or pseudo-single domain grains of magnetite interact. Similar results were obtained on one savannah sparrow (Emberizidae: Passerculus sandwichensis) head and one indigo bunting (Emberizidae: Passerine cyanea) head. When some bobolink heads were subdivided, most or all of the magnetization was concentrated in the area from the nasal cavity to the orbit. Light microscopy and histochemical tests confirm the localization of inorganic iron in this region.