The hens will do it all. Hens have been raising chicks on their own in all kinds of weather for millions of years - that was before heat lamps.
I've had hens hatch and brood chicks down as low as -5C.
When it got colder than that I did hang a heat lamp above the food and water so when the chicks came out from under the hen, they could eat and drink in comfort but not in warmer conditions.
At 4 C or above, the chicks run around in the cold for a while and then run back under the hen periodically to warm up and then they are out running around again. They really aren't that fragile. They do need a warm spot (under the hen) but not around the clock.
I've had hens hatch and brood chicks down as low as -5C.
When it got colder than that I did hang a heat lamp above the food and water so when the chicks came out from under the hen, they could eat and drink in comfort but not in warmer conditions.
At 4 C or above, the chicks run around in the cold for a while and then run back under the hen periodically to warm up and then they are out running around again. They really aren't that fragile. They do need a warm spot (under the hen) but not around the clock.