Magnetic position determination by homing pigeons?

How pigeons return home from unfamiliar release sites is a long-standing puzzle in animal behaviour. Walker (1998, 1999) has described a "vector summation model" which "identifies a novel coordinate that pigeons could use with magnetic total intensity to determine position". The model is not applicable in a magnetic field generated simply by a geocentric dipole, but requires a field perturbed by higher-order sources. Tests are devised to simulate the addition of both regional and local magnetic anomalies to a geocentric dipole field, and to calculate the directions of the home loft from a number of release sites. The results indicate that a pigeon would be unlikely to derive useful information from the model.