John Alcock, Animal Behavior: an Evolutionary Approach, Sinauer Associates, 1998

In both experiments (A and B), homing pigeons were exposed to a specific scent (benzylaldehyde, [BA]) blown by fans into their home loft from the northwest. In each experiment, one group of birds (solid circles) was permitted to smell BA at the release site. Under the olfactory hypothesis, these birds were predicted to fly to the southeast, and many did. Another group of pigeons (open circles) had their sense of smell blocked by an anesthetic before release. They could not detect BA, and so could not be deceived into "thinking" that they were northwest of their home loft. These pigeons flew directly toward home, which was to the west in experiment A and to the northwest in experiment B. The arrows within circles show the mean initial orientation of each group of pigeons.