I am pretty certain temperature varies with temperature and core temperature of the bird. It does not normally cool to freezing and feels a little warm even when a bird is walking on snow. Otherwise lower ambient temperature mean cooler feet. When a bird is exerting itself or otherwise stress by heat, then more blood is shunted to the feet making them feel warm where the heat can then be "dumped" into whatever the bird is walking on. This is like a dog.
You can purchase a relatively low cost devise to detect surface temperature based on infrared light coming off feet. You also need to better define what a foot is and what parts are being measured.
Data like you are looking for likely does not exist in the literature for chickens. You might find it for wild fowl living under extreme thermal conditions. For giggles I will run to lab tonight and fetch my reader to see if it works. You peaked my interest.