Below is a post from the 'old fashioned' broody thread from last February , I thought it may be helpful for some folks on this thread. Remember that this is just based on my experience in our coop with our flock dynamics, but I believe it may provide a good base for someone to begin evaluating their own situation with....
Originally Posted by fisherlady
I agree with Lady of McCamley, it is very possible to do, but it totally hinges on the hen's mentality. Many hens are tricky about fostering... it isn't their fault, and it doesn't make them bad broodies, it is just a hormonal thing, this seems more pronounced when the hen is a newer broody (only one or two hatches). I have found my more experienced broodies seem much less phased by odd things
Remember that the natural progression of brooding is (from my observations)
a). Pre broody.... the hen's hormones are starting to act up, they tend to be vocal and grumpy, mine often walk around emitting a low 'clook, clook, clook' noise for a few days during this phase. This is also the time they start hogging a nest box during the day and gathering eggs when they can. They often still roost at night during this early phase, so it is easy to miss if you aren't around them for extended periods during this time. I have come to recognize these as an early warning sign.
b) Setting broody... the hen has committed to setting full time on the nest now, her hormones have fully kicked in and she wants eggs no matter what! She may still take in extras she can find so it is up to us to prevent that during the setting period. While setting she will instinctually turn the eggs multiple times a day and shuffle their position in the nest to what she feels is the best benefit to the eggs. During this time she will only leave the nest one or maybe two times a day (if even that often) during which time she will be the whirling dervish queen of speed scratching, dusting and drinking.
c) Hatching broody.... it seems that the last few days of a normal clutch the hen senses the increase in movement of the chicks in the egg and you will often hear her softly purring and clucking to the chicks. This period seems to help prepare her for the sudden appearance of the newly hatched chicks. Although a broody can't count the number of days she has been setting she does recognize the changes in the vibe from the eggs as hatch nears.
d) Mama broody.... usually this kicks in on day 2 after the hatch (give or take 24 hours) At this time the hen decides it is time to leave the nest and care for all of the little ones which are out and about. She will often give up on late eggs, sometimes even viable ones, because the instinct is to care for the already hatched, not hope for more. I re-purpose an old phrase to explain this..... 'a chick in the open is worth 2 in the shell' Some folks don't understand the importance to general survival that this is... and it is also the reason that staggered hatches are so dangerous to the hatching process.
I have encountered problems when messing with the normal progression of brooding, but the problems can be minimized and/or avoided when you understand that they are possible to begin with.
a) hens can't count days, but their hormone levels evidently do fluctuate and some hens just don't do well with being suddenly presented with chicks when they have only been in 'setting' mode for a short period. I had a hen who had been a great broody before when I gave her a group of eggs which were due to hatch only 10 days into her brood. She just didn't transit to hatch mode and the first two eggs which hatched she suffocated because she just was sitting too tight.... she just wasn't understanding that hatch time had kicked in. We removed the remaining eggs and gave them to another broody and we replaced them with fresh eggs to this hen. She happily sat on them for the 3 weeks and hatched 6/6 and was an awesome mama to them. Her failure to short hatch didn't mean she was a bad broody, her hormones just didn't adjust in time. I now normally wouldn't short hatch a hen with less than 2 weeks of brood time to avoid the possible problem.
b). some hens are great fosters but others can seem very confused when chicks suddenly appear without the 'hatch mode' preparation time. These hens aren't bad broodies, they just need a bit of time to transition to mama mode. These situations need very close monitoring to help lost chicks get back under the broody and also sometimes some chick protection, a confused hen may peck at a chick and hurt or scare it during introduction. Simply cupping the chick under your hand and sliding it under broody without her even seeing it or having the chance to peck at it can drastically reduce stress to both broody and chick. Broody will hopefully begin getting into the mama mode with just hearing the chick under her for the first 12-24 hours.
c) mixed foster method. This is my preferred and I have done well with it. A hen who has recently hatched (within past 2 days) will usually readily accept more chicks with very little fuss. Her natural hatchers have already helped her transition through the hatching mode to mama mode and hens can't count, so more chicks just appearing with her own seems to be pretty simply and acceptable to her. The only risk is if the new ones are a different color. Some hens do seem to show preference to their own if they are old enough that she was out of the nest with them. Some hens may reject an odd colored chick. So watch for that risk. If mama has 6 yellow fluff balls and you try sneaking in a black australorp chick she may object.
Now remember that these stages and situations are just some general observations in my coop, and their are exceptions to every rule... I have a hen who will have nothing to do with chicks which aren't hers (a rarity in our coop, but she is a character) and I have a couple of hens who will adopt anything that cheeps, even if it is a much older chick.... some hens will adopt chicks at the drop of a hat when they haven't even been broody or only broody for a short time. We are totally at the mercy of the hen's personality on this.
. Our faithful Janeway, with her own chicks (maybe a week old) and a group of almost 5 week olds she decided still needed a mama when their own broody went back up on the roost (see the little white head poking out from under her right side? that is one of the older group). Janeway kept that older group with her until her own were 6 weeks old, in fact, the older chicks abandoned her more so than the other way around, I think she would have kept them longer yet, even though they were nearly as big as her by the time they were 10 weeks old!