With a mortar and pestle coarsely crush rosemary and in a small bowl stir together with butter, thyme, zest, and salt and pepper, to taste, until combined well. Spoon mixture onto a sheet of plastic wrap and form into a 4-inch long log. Chill compound butter, wrapped well in plastic wrap, until firm, at least 30 minutes, and up to 3 days.
Preheat oven to 475 degrees F.
Discard gizzards from birds and trim necks flush with bodies, if necessary. Rinse birds inside and out and pat dry. Starting at neck end of each bird, slide fingers between meat and skin to loosen skin (be careful not to tear skin). Cut butter into 16 (1/4-inch thick) slices and gently push 4 slices under skin of each bird, putting 1 slice over each breast half and thigh. Tie legs of each bird together with kitchen string and secure wings to sides with wooden picks or bamboo skewers.
Arrange birds in a flameproof roasting pan large enough to hold them without crowding. Gently rub birds with lemon halves, squeezing juice over them, and season with salt and pepper. Roast birds in middle of oven 30 minutes (for poussins) to 45 minutes (for Cornish hens), or until an instant-read thermometer inserted in thickest part of a thigh (be careful not to touch bone) registers 170 degrees.
Transfer birds to a platter and loosely cover with foil to keep warm. Add the wine to roasting pan and deglaze over moderate heat, scraping up brown bits. Transfer jus to a small saucepan. Skim fat from jus and simmer until reduced to about 1/2 cup. Garnish birds with herbs and serve with jus.
