When a chicken setting has an egg hatch under her you then have 3 days to give her any baby chick you can grab and she will take about any as long as they look close to what she had hatched. Like in Size and breed. If the setting hen is hatching chicks you can't give her geese or turkey poults. If the hen hatched ducks you can give her chicks and she will take them sometimes but not always true. You can get hen to take chicks sometimes but to do it you have to put them under her after dark and they sleep together and wake up together. It is very difficult to get her to take chicks if eggs are not hatching under her,.
Not always true. Some do well with them and some maybe not so well. I have a hen here who hatched out ducklings for a friend last year, once the ducklings hatched my friend came to pick them up to raise them herself and my hen was beside herself. I ran to the feed store and got her 3- 2 week old mottled cochin babies. She took them right away. Different hen same scenario wouldn't accept the chicks. It all depends on the hen and really how long she has been sitting already. If she has been sitting on eggs or nothing for a few weeks your chances of her accepting the chicks are better then if she had only been broody a few days. It is also reccomended you give her the chicks at night when she is bed down for the night.
My neighbor tried this with urban store chicks that were 1 week old and kept sticking them under her broody and she finally came out to 3 of the 5 dead.
Week old chicks are a little bit too old to try it with IMO. Obviously you have to stick around and observe the hen's behavior to know whether it will work or not. The hens I did it with were a silkie and a salmon faverolles and both were crazy broody.
Here's the SF with her store bought chicks (and her chicks from an earlier hatch):
Here's the silkie with hers (she hatched the turken, the polish were the store bought):
I haven't read the other responses, but here is my only experience with this.
I had read on BYC that you should do it at night and sneak the chicks in your sleeve up to the hen so she doesn't see, and put them under.
But I decided to just simply put them under her during the day and not hide anything.
She pecked them and I got her outta there as fast as possible. Had to go back to the feedstore to get more chicks because the two that I had bought JUST FOR THE BROODY as a gift were standing doing the 45 degree angle leg thing leaning into each other in shock after I took the hen out.