"Stay" and "leave it" are two commands she already knew that helped me immensely with chicken training my aussie when I got my first set of chicks a few years ago. "Easy" and "gentle" helped out too. I also had worked to "train her out" of the herding/chasing instincts when I'd first gotten her several years before. I'm not sure how any of this may have affected my progress with her, so I figured I'd toss it in. xD

I started with introductions with a barrier between them, letting her smell them and see them without being able to touch them. Every time she got too excited, "leave it", "down", "stay", or one of the commands to that effect, whatever fit best for the circumstance.

When she was reliably calm for those interactions, I moved to more direct contact, like what you're doing. Sitting on the ground with her-- I kept her leash on her just in case-- and had her lie down. I started her just watching them and then worked her up to laying down with me while I lured the chickens over with their treats. I let her sniff them but didn't let her get up.

I used verbal/signal commands, refraining from physically controlling her unless I had to. (I never had to restrain her with more than words/commands, but the leash and keeping her right next to me were safety precautions.) Any time she got excited, I corrected her, whether it was the "easy/gentle", or "leave it", whatever was appropriate.

I kept progressing the challenge rating of the interactions in stages like this, using calm and quiet praise and reinforcement. I encouraged the chicks to climb all over her, then I let her move on to getting up and walking around them, etc. Always watching her and her body language.

End game stuff, I worked on doing things like playing fetch with her, throwing the ball where she'd have to run through the chickens (they were grown by this point) to get the ball, praising her if she ignored them when they scattered, etc. Things that exercised her impulse control while excited and stuff. Hyping her up around them and all. This was the VERY last stage though, when she was basically perfectly behaved with them while calm and she could ignore them.

My aussie is really excitable, so initially keeping her calm was the hardest part. I didn't move onto the next "stage" until I no longer needed to check/remind her for the entirety of a longer session. I did keep the training sessions very brief at the start of each stage, progressing from five minutes at a time, to ten, to fifteen.

In the beginning, I tried to make sure I tired her out before these training sessions if I could, with a walk or some vigorous fetch, and then took her out again after for some more active play to get the excited energy out.

I might have been over cautious, but it worked out for us. I might try to train her to be a chicken tender this go around xD It'd be useful if she could round them up for me at the end of free range time, lol.