While vegetable oils are often used as a wood finish; however, it is better to avoid using most vegetable or cooking oils completely. They are not the best bird safe wood treatment. The main reason for this is that the vegetable oils tend to coat and thicken on the surface of the wood and form a cloudy layer, which may peel off after a while. Also, the wooden surface can get discolored when you apply the vegetable oil. Vegetable oils become rancid after some time and may cause your birdhouse to smell quite bad and they are also highly flammable.