I'm still on page one.. but agree with others that grown birds do not need added heat.
Their heat source is internal and they are insulated by wonderful down which they tuck their beaks and combs into during the night.. the feathers also cover their feet when they hunker down on roost.. they're cozied up inside warm down jackets and mummy bags relatively.. Keeping them dry and not having excess moisture build up inside the coop and settle on them creating frostbite.. is key.. keep it dry.. not warm.. they are equipped to handle it.. making it warm makes it more challenging to adjust when going outside plus what happen when there's power outages and the birds are suddenly without what they've depended on.. they are creatures of habit and will do as they've been accustomed to so they may be at a serious disadvantage if they've never learned the value of huddling.. they don't think their way out of situations well.
If I were going to use either.. it would be a sweeter heater type, wall panel, or something actually made for animals.. there are heater pads with hard wire protecting the cord from chewing etc.. also water proof while plugged in. That being said.. heat lamps have been used plenty safely by farmers for decades.. there is some risk to everything in life. As much as I also poopoo and dislike heat lamps.. I do recognize their usefulness..
For chicks.. a standard heat plate is by far superior to both a heat pad and a heat lamp in MY experience.. in electricity savings, adjust-ability, ease of use, reliability, and on and on... I almost killed a whole brood trying to manipulate a heat pad with some plastic wrap.
Think about nature.. chickens stay where they can get under mum while they still need her heat. Once they mature they ditch mum and never tuck under another to warm up again. The only time, they may need help with thermal regulation might be during times of illness.. like if your flock has bronchitis, coccidiosis, or other illness.. then the body is challenged in many aspects of function.
With chickens.. straw kinda sucks.. poos sit on top and then get matted down more so than with shavings. Straw is cheaper than shavings but mite carry some mites.. There's no perfect answer.. which one can you keep the driest and work with the easiest.. I did use straw aiming for semi deep litter in my coop and it's fine as long as I turn it regularly. Storing hay invites rats more than shaving do.. so far for me.. So many many factors.. are at play.. noting that insulation is not really one of them.. A thick layer of either should be about the same.. Landing off of roost.. I haven't yet decided if shaving or straw is more likely to cause a splinter or other type of foot penetration or increase likely hood of any bumble foot type infection. Using both, with 4 foot roost and heavy breed large fowl haven't had issues YET.
Just sharing thoughts and agreeing with others that
healthy birds( with plenty of nutrition not diminished by excess low nutrient high energy sources such as scratch or corn), don't need
added heat provided they are given well
ventilated, draft free (no feather blowing wide open), and
dry shelter.
However.. If I set up an experiment and discovered that they
safely enjoyed the added creature
comfort.. AND also had the resources to provide it.. uhm.. YA, that's the value of living in a first world country and owning PETS that aren't only for food!!