Smells that are known to deter or repel birds include the scent of weasels and methyl anthranilate, a chemical used as artificial grape flavoring. Birds have delicate respiratory systems, and certain fumes innocuous to humans can be hazardous to them, so caution should be applied when using gaseous bird repellents.
Most bird species have a sense of smell, and in some cases this sense is very well-developed. As of 2015, most bird repellents still work on the now-disproved theory that birds cannot smell, and therefore rely upon sound or sight cues, but at least one repellent on the market relies upon bird's aversion to the scent of methyl anthranilate.
Few other odor-based repellents are commercially available. While scientists know that weasel scent can deter blue tits from entering nest boxes, this odor is not commercially available as a repellent. Similarly, Eurasian rollers behave cautiously when presented with the foul-smelling liquid that their chicks expel in the presence of predators. This odor is also not used in any commercial products.
Birds are especially susceptible to harm from air-borne toxins. They have a system of air sacs throughout their bodies, storing oxygen as needed and lightening their bodies for flight. This gives them great endurance at the cost of being more vulnerable to toxic vapors. Pet birds in enclosed spaces are more prone to being damaged or killed by fumes, but even wild birds can be harmed by chemicals such as heated Teflon.