Birds do not experience the sensation of heat when consuming spicy foods due to the fact that they are generally not sensitive to capsaicin, the compound responsible for the spiciness in chili peppers. While capsaicin acts as a deterrent to mammals due to its heat-inducing properties, birds are largely unaffected by it.
In fact, some studies suggest that birds may even be attracted to capsaicin-containing peppers. Birds lack the receptors that mammals have, such as TRPV1, which are activated by capsaicin, so they do not experience the sensation of heat or pain when consuming spicy foods.
One theory for why birds are not deterred by capsaicin is related to the dispersal of chili pepper seeds. Birds play a significant role in spreading the seeds of chili peppers, as they consume the fruits and then excrete the seeds intact, helping the plant to propagate. Since birds are not affected by capsaicin, they can freely consume chili peppers without being deterred.
This unique relationship between birds and capsaicin has led to the evolution of some chili pepper species to produce fruits with higher levels of capsaicin, as it provides an advantage in seed dispersal by attracting birds while deterring mammals that might otherwise consume the fruits without aiding in seed dispersal.