When/how to merge two very young flocks?

Since these will both be in a brooder under heat, it will depend on your set up. how big is your chicken coop and run? The easiest would be if you were able to wait until both groups are old enough to go out into the coop and introduce them there on neutral quarters. While you still have them in the brooder, I would keep them near each other so they can see and hear each other too. Inside the coop, allow for areas that they can run away, escape, get behind, barriers etc. If that doesn't work, then I would put the first batch into the coop, and when you are ready for the 2nd batch-I would put them in a cage inside the coop for a few days to see each other and get used to each other. Then on a day where you can watch them for awhile, let them out together. If you free range-even better as they have more room to run and escape as needed. I hope this helps!

 

If you can split your coop and run into 2 sections separated with chicken(or other) wire that would be the smoothest and easiest.

You could put a small opening in the separation wall just big enough for the smallers to go back and forth, by the time the smallers don't fit the opening, or the opening has been enlarged enough to fit both groups of birds, they should have integrated themselves.

I did this on a smaller scale with 4 smaller chicks in a dog crate with a small entry right in the coop with the olders, it worked very well once I got it figured out.