The Teflon gives off toxic gasses when heated. Birds (such as chickens or other poultry) are very sensitive to airborne toxins and can die from the exposure to such fumes. This can happen quickly. There have been a number of reports of small flock owners finding their entire flock dead in the morning after installing this type of light. A large scale poultry research facility has experienced the death of all 2400 birds over the course of a few days following the changing out of regular heat bulbs for the rough service lightbulbs. Sylvania and GE both make these rough service lightbulbs and are readily available, often close to the heat lamps in stores. GE has no warning labels on their bulbs, but Sylvania does have a sticker warning that the bulb can be harmful to poultry.
The Teflon starts to off gas as low as 325F and possibly lower. Many light bulbs will reach better than 500F, creating a lot of initial off gassing when first turned on. This cloud of toxic gas is what kills the chickens and other poultry in their coops on a cold night.
On a side note, this is the same Teflon used to coat the common household pans, with the same off gassing into your kitchen where you and your family are breathing the air!