I'm not trying to be critical, but I think this is why you had some problems. You more than likely had a lot of moisture in the coop with them being "cooped up" - and heat lamps add to that. Moisture causes frostbite.
We all have different management styles, but I don't use a heat lamp outdoors for many reasons - one the moisture, two if you lose power then the birds aren't used to the cold and the drop in temperature can kill them, and three the risk of fire.
I let my birds out unless we're getting >6 inches of snow, regardless of temperature.
It is likely they grew slower, they were probably using some calories to keep warm instead of bulking up.
I don't take it as critical, you never learn if you are never told the truth.
The thing is they are in a 24' x 24' building ok well it's a garage, and the only reason I had put the light on was I lost a very important cockerel and the night he died it went from 30 degrees to at least -8 if not a little lower it was -8 that moring, there was no light on that night, and then stayed in the negatives for more than two weeks. Most of January into February was so very cold. It would say 0 but felt more like -12.
Plus I guess I should have elaborated on the whole situation. There were only a few pullets out of the lot who grew very slowly or got frostbite. In the hen pen there were 30 hens. As for the young ones I had
7 Sussexes-both breeder stock and hatchery stock did overwhelmingly well-they grew like the chicks in the summer if not faster and no frostbite-so doesn't matter if it came from a breeder or hatchery stock
3 Welsummers slow growing a few got little frostbite on nails they are still small
Leghorns which got absolutely no frostbite and they never went outside except for maybe a few minutes every now and then stayed small,but aren't they small to begin with? I didn't think they were out of the ordinary size wise and when I say they didn't go out they only didn't go out by choice
Marans not directly affected in growth or frostbite
a chantecler got frostbite on one toenail and very slow growing still very small
one EE who really got the brunt of everything, poor thing got frostbite on every toe plus was the slowest growing of all. She did go outside all the time and would stand on one foot at a time, I do have to say in the last two weeks she shot right up is now the tallest chicken even next to the English Orps plus her legs have doubled in size.
ALL my chickens were DIVAS! They refused to go in the snow I had to put hay down in their outside pen before they would go out side. They would stand on their ramp and just stare. I did have 1 or 2 hens decide to try the snow in January but it was only the one time then they ran right back in!
I used the deep litter method and back in Dec it did get wet but we cleaned it then added a ton of shavings, hay and shredded paper and then it didn't get wet again.
I get that they probably grew slower because they were putting it towards heat but it somehow doesn't seem right-I guess I mean is it doesn't seem the best choice or optimal choice.
My cockerel was in the roo pen also got frostbite on a couple of his toenails but no other roo got this on their toes. The roo pen was NEVER wet so why did my cockerel get the frostbite? The roo light only went on when I saw them shiver. They never went outside.
I did not plan on using a heat lamp, again the first time I put it on was when it was so cold your throat would hurt from the cold when you went outside. It gets pretty cold here sometimes, so much so they don't even let the kids out at school during those days! We sometimes joke that we get colder than half of Canada he, he, he!
sorry hope that wasn't too long