My Rhode Island Red and Ameraucana purr like a cat. They seem to be at peace when they do this, so it must mean they're happy.
Chickie, my 4 month old hen, has always purred if I talk to her and tell her how pretty she is and how much I love her. I do think it means she feels safe, peaceful and content.
I have a routine with one hen where I hold her and tuck her under my chin. She will usually stretch her neck and lay her head against the hollow of my collarbone (warm rubbery comb and all). That's when I scratch her neck under her earlobe and she closes her eyes and purrrrrrrrrs. She knows where her bread is buttered!
