A bi-coordinate map sense based on geomagnetic information was first suggested by Viguier, almost 130 years ago, but it did not receive much attention until the beginning of this century. The magnetic map hypothesis in animal navigation has attracted an increasing amount of attention during the last decade and magnetic navigation has been demonstrated in several animal species from widely separated taxa.
In a recent paper by Boström et al. 2012, published in Ecography, we investigated the global geomagnetic prerequisites for geomagnetic bi-coordinate navigation in order to pinpoint possible problematic, as well as suitable, areas for migrating animals. We analysed the angular difference between isolines for inclination and total field intensity on a global map divided into 3° lat. × 3° long. squares and produced a map illustrating different regions of the earth defined by different angular intervals. This map revealed four vertical bands of little or no variation between isolines (‘no-grid’ zones) in the northern hemisphere as well as a few areas in the southern hemisphere. Most part of the globe show intermediate angular difference (2-30º), but there are also large regions both at northern latitudes and in the Pacific, Indian and south Atlantic Oceans where the two isolines form a clear grid.