For the first 10-14 days of its life a pigeon will be brooded by its parents, so a rescued one will need a gentle source of heat. Do not place them in front of the fire or in direct sunshine as they can overheat and die.
Safe heat can be provided by:
An electric heat pad set on low.
An angled desk lamp with a 40 watt bulb or (ideally) a 40 watt nightlight red reptile bulb.
A microwaveable “Snuggle Safe Pet Heatpad” available from Pets at Home and from Amazon UK. The advantage of these is that they provide heat for an extended period, the disadvantage is that they can only be reheated when they are cold, so there would have to be a second heat pad or an alternative to provide the baby with constant warmth.
A hot water bottle wrapped in a towel (this would have to be changed every 2 hours)
A micro hottie…keeps warm for up to 5 hours.