Planning Integration of new chicks and have a few questions

[Beamerz]

I read that awesome post from 2016 on "

Reasons for Tossing Out Your Indoor Brooder and Start Raising Your Chicks Outdoors"​

That is an awesome post and hinged on some ideas that I was trying to conceptualize . I've got a 3 yr old flock of 14 in my coop with 2 roos ( so far so good) one was raised in the flock... I just ordered 15 chicks "early" as I was afraid if I waited ... well who knows what could happen..? no one I know I'll just leave it at that.... but I ordered earlier than I would have as I live 41 degrees north lat.. in the east... and it will dip down to freezing at night until mid April if my long range forecast is right.
Now I love this idea that was put forth in the above post ( you can and should search it)... but I was wondering .. I've built a large brooder 4' x 6 'ish covered in hardware cloth so my chicks will be in view of the chickens in the coop.. it will have a separate door to a separate outside run that can be included or excluded from the big chickens.. I was wondering if I might consider using "plant sprouting mats" ( I have a few of them and some are larger than regular heating pads but I don't think they get much hotter then maybe 80 or 90 degrees.. I have to test them... but I thought I could fasten them to a lidded plastic tote box (outside of it) and cut a door opening for them so they would have a sheltered warmer than maybe mid 30 's place to sleep or warm up, but it would be on the floor of my coop ( inside the 4x6' screened in box with wire doors that lift up on top for access and at least one small sliding door for outside access. It's only 24" high but if I put a luan bottom on it and prop it up on milk cases only when in the house I don't have to bend down to service the "circus" that the chicks create.. they are so cute... (addictive we know) ... So my question is...... I was ( before reading that post I cited) thinking I'd have to keep them in the house for weeks and weeks.. but wondering if anyone in the north has and idea of at about what size, weight , point of development chicks might be able to adapt to normal life without too much risk? thanks for reading and your comments... Also as an aside...as this is my first time raising chicks from a hatchery... ( My daughter had raised the original flock and I have raised chicks under hens ( well the hens have ;-) but never direct from hatchery.. and in winter... But I have introduced (in a limited way) one of my former mothers to the new chicks who was originally "the lowly girl" until she became a mama.. last summer and then she rose in the flock but this little bantam was the best darn Momma... and she wants to get in with the chicks.. and I'm trying to condition a relationship because I don't know how they will fair in the spring without a Momma as we do a lot of free ranging an predator's are present.. I had been thinking I can't really bring her into the house but if I can figure the heating outside part out I think she'd hop into that brooder BUT with so many chicks that 's not fair.. I have another back up Mama but even typing this I am expecting people to be shaking their heads.. your going side to side aren't cha?
Thoughts on my wanting to introduce a former mother/s? Would love to hear from the seasoned keepers...! Thanks for being here.. in my backyard.... chickens

 

[saysfaa]

They stop needing a place to warm up when they are fully feathered - about six weeks. Somewhat earlier or later depending on things like whether they have the slow feathering gene or the fast feathering gene, whether they have normal feathers or not (silky, frizzle, etc), and whether they are adapted to cooler temperatures (exposure to cooler temperatures results in a different chemistry in their muscles - I think muscles, I should probably look it up again. Anyway, it is analogous to the brown fat vs white fat in mammals. If you want to look it up in google scholar, a good search term is "non-shivering thermogenesis")

You might also consider a wool hen and/or a wool hen along with your plant heating pads. And please let us know the results of whatever you try.

 

[Beamerz]

saysfaa thank you so much for your reply and comments.. very helpful... I have them in a room with a wood stove and it's pretty warm so I'll try to move them into a cooler room in a week or so and gradually reduce the temperature as they continue to feather so the transition will not be jarring.. and their muscles as you say can adapt.. Thanks for that great factual info.. love it. And I will experiment with the plant heating mats and let you know what I can lean. I bought one of those point and shoot thermometers so I'll be able to do some good experiments.. and share my results.. Chickens Onward!!​

Thanks again!