I have had at least 4 broodies that i can recall (i have dozens of hens go broody every year so may be forgetting someone), that accepted chicks (and so likewise chicks that accepted real feathered mamas) when the chicks were 3 weeks old. And Many other broodies that accepted feed store chicks up to one week old.
This includes a first-time broody just this past october 2020. She already had her own 3 week-old chicks, & then immediately accepted another hen's 3 week old chicks too. I wont go into the reasons i needed to Try and see if those broodies would accept older chicks. But i believe i would have additional sucesses with some other broody hens too, if i had tried. Essentially, you wont know unless You also try.
The first important step is to place the chicks under her at night. Dont let her see the chicks at all; put them behind her up under her tail so they can snuggle in. Watch/listen VERY carefully to make sure she is ok with feeling the chicks underneath her. Because alas, ive also had one broody that started immediately killing feedstore chicks i placed beneath her at night. Like u wish to do, i removed eggs she had been incubating for approximately one week, & replaced her eggs with chicks. She wasnt fooled. She immediately killed one chick and slightly injured another before i could rescue those remaining. And that hen is a GREAT mom when she hatches her own. I have another broody that is ALSO a great mom as long as she hatches her own eggs. She never killed introduced chicks, but she didnt accept them either. Sorta made them stand in the cold corner until their REAL mom came back to get them. (That would be me). You will also need to make sure you are up before dawn the next morn after you give her the chicks, to make sure she is still accepting. If she tidbits the chicks to show them food, & lowers her body to the ground to allow them to go underneath her for warmth, all is well and everyone is happy So to sum it up, i agree it is possible, & probably likely, that your hen wont accept the chicks. But it is also Possible that she will. Just know that you are taking a certain risk by trying, & be ready to immediately remove the chicks if your broody rejects them.
Yay initial introduction was 100% sucess! Re another hen(s) being with the mama & her chicks, also be aware that sometimes other hens will also try to harm a broody's chicks. Usually depends on how high the broody ranks in the pecking order. If broody is top hen or near the top, the other hens will usually leave the chicks alone. If your broody ranks low in the pecking order, u Might have to give her a separate safe place to raise her brood. At 1 week old, the chicks are at least qable to quickly scoot under mama hen for safety. The hen on right in your pic was definitely curious to see what your broody was hiding underneath her. Curious is ok! Just observe and make sure the chicks are safe from the rest of the flock. Your broody hen in pic definitely looks very happy & protective to have her some chicks! And now you now That girl is a broody that will readily adopt chicks. Please do keep us updated, & post more pics when u can!
