Step 1Heat the oven to 450 degrees.
Step 2Combine the tarragon, chives, parsley, dill and garlic in a small bowl. Add 1 teaspoon salt and toss to combine. Add one-quarter cup (one-half stick) butter and mix to make a paste. Carefully slide fingers between flesh and skin of each poussin to make a pocket from the breast over the legs without tearing the skin. Spoon one-fourth of the herb mixture into each pocket, with a little in the cavity. Pull the skin taut and carefully massage the herb mixture to distribute it relatively evenly. Season the birds inside and out with plenty of salt and pepper. Truss the birds if you like, or leave untrussed for a more evenly crisped skin. Rub the remaining butter on skin to coat.
Step 3Arrange the poussins on a rack in a shallow roasting pan. Transfer to the oven and immediately lower the heat to 375 degrees. Roast until the skin is crisp and juices in the leg run clear when pricked (an instant-read thermometer inserted into thickest part of leg should read 160 degrees), 35 to 40 minutes. Remove from oven and let stand 10 minutes before serving.