Searching by rules: pigeons' (Columba livia) landmark-based search according to constant bearing or constant distance.

Pigeons (Columba livia) searched for a goal location defined by a constant relative spatial relationship to 2 landmark. For one group, landmark-to-goal bearings remained constant while distance varied. For another group landmark-to-goal distances remained constant while direction varied. Birds were trained with 4 interlandmark distances and then tested with 5 novel interlandmark distances. Overall error magnitude was similar across groups and was large than previously reported for Clark's nutcrackers (Nucifraga columbiana). During training, error magnitude increased with interlandmark distance for constant-bearing but not constant-distance birds. Both groups searched less accurately along the parallel to landmarks than along the perpendicular axis. Error magnitude increased with novel extrapolated interlandmark distances but not with novel interpolated distances. Results suggest modest geometric rule learning by pigeons.