Actually both, the taste and smell of the acitve ingredient is irritating to the mucous membranes of only birds (like hot pepper juice is to people).

During the initial exposure to the Rejex-it product the bird sees the smoke/fog plume and associates this with the unpleasant sensation that it experienced.

Essentially the bird is being trained to associate an unpleasant sensation with a particular location this is why it takes a couple of treatments to achieve maximum results with the Rejex-it products because you have to train the bird to avoid a particular location. The birds will also learn to avoid the smoke/fog plume or the smell produced by Rejex-it products.

Depending on the species of bird it may take from one to three exposures to train the bird to permanently avoid any given area. I hope this answers your question and if you have any more please ask.