I agree that chicks are not as fragile as we seem to think they are. I think keeping them inside for too long or providing them too much heat actually handicaps them to a degree... by not letting them grow their feathers sooner or start building immunities to the outdoors. A broody hen starts them outside from day one.
I also think a heat lamp tends to complicate things (too much/too little heat, fire risk, light on at night when it should be dark). I love a heating plate or
MHP... it's so much more natural. Ambient heat from a light is very different from directly touching a warm heat source. Chicks choose when and how much heat they need and then decide on their own when they no longer need it at all. I raised my chicks last year in the coop starting at 2 weeks old (only because it took that long to finish building my new coop and brooder setup). Nights were 40s and days close to 60s and they always had access to a Mama Heating Pad, though they rarely used it.
I started early integration at 3-4 weeks by switching to a new divider that the babies could fit through but my older birds couldn't. When my chicks were 4-5 weeks old they made their own choice to join the big girls on the roost at night and abandon the heat source all together.
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This just proves to me that when left to their own devices, those little chicks can do pretty well and acclimate very quickly. Many other folks brood this way as well. But if you're using a heat lamp inside, it always seems to take longer for them to grow in all their feathers and get used to being a real outdoor chicken.