1) Preheat oven to 220C/Gas 7.
2) Line two baking pans with foil for easier cleaning. Set a baking rack in each pan. Split each poussin and butterfly open. Place three poussins in each baking pan. Liberally season the poussins with salt and pepper. Mix the orange concentrate, balsamic vinegar and olive oil and pour evenly over the poussins. Sprinkle with rosemary and roast for 15 minutes at 220C/Gas 7, then reduce heat and roast for 30 minutes at 190C/Gas 5.