One animal carrying two kinds of muscle with doneness targets 25-30F apart. Breast is finished at 155F, thigh isn't finished until 175F or better, and in a normal roasting position the breast is the part closest to the heat. Every technique for roasting a whole bird is some way of dealing with that.
A supermarket bird is a broiler slaughtered at 7-9 weeks, 3.5-5 lb dressed, bred so the breast is the largest single thing on it — around a quarter of the carcass weight. That breast sits high on the keel, exposed, fast-twitch and nearly fatless. The legs hang low and to the sides, dark and worked, with the heaviest connective tissue at the hip and knee. Set the bird breast-up in an oven and the muscle that needs the least heat is the highest point in the box while the muscles that need the most are shielded by the body cavity. Over all of it is skin, roughly 40% fat over a collagen dermis, and skin is the one component that wants hard, dry, sustained heat from the start.
Three clocks run at once and they don't agree. The breast passes its point of no return at 155-160F, when actin denatures and the fibers expel water. The thigh needs 175-190F sustained long enough for collagen to convert to gelatin, and at 165F that conversion has barely started. The skin needs its surface water driven off and its fat rendered, which requires surface temperatures well over 212F — a low, gentle oven is the enemy of skin. Carryover in a 4 lb bird runs 5-10F after it leaves the oven, so the breast comes out at 150-155F. The two fixes that actually work are both geometric: spatchcocking (cut out the backbone, press the bird flat) moves the legs to the hot outer edge and drops the breast into the shielded middle, and it cuts roast time from 60-75 minutes to 40-45; starting breast-down uses the bird's own body as a shield until the legs catch up.
Flattening puts the legs at the outside where heat is highest and the breast in the middle where it's lowest, which is the only move that reverses the built-in geometry instead of compensating for it.
HowKitchen shears down both sides of the backbone, press the breastbone flat, skin up on a rack over a rimmed sheet. 450F, 40-45 minutes for a 4 lb bird. Breast 155F, thigh 175F+ at the joint.
The two-temperature problem disappears entirely if the two kinds of meat aren't attached to each other — legs get their long time at 185F and breasts come out at 155F.
HowTwo breasts, two thighs, two drumsticks, two wings. Legs and wings in at 425F for 35-40 minutes; breasts join for the last 25. The back and neck go straight into a stockpot.
For the first two-thirds the breast is shielded by the body and sitting in the cooler bottom of the pan while the legs take the direct heat, so the gap narrows before the skin gets its turn.
How425F breast-down for 30 minutes, flip with wads of paper towel, 25-30 minutes more skin-up. Breast 150-155F, thigh 175F.
Salt dissolves myofibrillar proteins so the meat holds more water under heat, and open air in the fridge pulls moisture out of the skin, which is the prerequisite for crisping.
How0.75-1% salt by weight, about 1 tsp Diamond Crystal kosher per pound, inside and out. Uncovered on a rack in the fridge 12-24 hours. Do not rinse it off.
Stuffing is a heat sink packed into the coldest, slowest part of the bird, and because it's soaking in raw poultry juices it has to reach 165F itself. By the time a cavity full of bread gets there, the breast walls around it are 175-185F and the outer half inch is well past that — you've traded the entire breast for the stuffing. The loss is water expelled by denatured actin, so it doesn't come back with gravy or resting or slicing thin. Stuffing also blocks air from moving through the cavity, so the inside of the breast has to cook by conduction from the outside in, which is the slowest heat path available and widens the gap further. Bake the dressing separately with stock and some pan drippings and you get the same result without spending the bird on it.
3.5-4.5 lb is the size that cooks evenly; past 5 lb the mass gap between breast and leg widens and the breast overcooks worse no matter what you do. Air-chilled birds have drier skin and crisp better than water-chilled ones, which absorb water in the chiller tank — the label states retained water percentage. Watch for 'contains up to X% solution', which is injected brine; that bird is pre-salted and will steam. Skin should be unbroken over the breast with no dried or gray patches, and the legs should still move at the hip. Take the giblets and neck home: the neck and back are the two most gelatinous parts of the animal and they're what makes stock set. Figure about 1 lb of raw bird per person for a meal with leftovers.
Doneness: Breast 150-155F when it comes out, coasting to 157-160F. Thigh 175-185F measured at the thickest part beside the joint without touching bone. Rest 15-20 minutes before carving. If the thigh hits 175F while the breast is already at 165F, the bird is still worth eating but you left about an ounce of water per side in the pan.
A 4 lb bird at 425F takes 60-70 minutes breast-up, or 40-45 minutes spatchcocked at 450F. Rough rule for a whole bird is 15-18 minutes per pound at 425F, but weight is a poor predictor because shape drives heat transfer more than mass does. Use a thermometer: 150-155F in the breast and 175F+ in the thigh, and start checking 15 minutes before you think it's due.
Skin is about 40% fat over a water-heavy collagen layer, and it can't crisp until that water is gone and the fat has rendered out. That takes surface temperatures well above the boiling point, which means a hot oven — 425-450F — and dry skin going in. Salt it and leave it uncovered in the fridge overnight, don't cover the pan, don't baste with liquid, and don't crowd anything else onto the sheet. If the bird was water-chilled or brine-injected there's extra water to drive off before you get anywhere.
For evenness and for time, yes, and by a wide margin. Flattening the bird puts the legs at the outer edge of the pan where the heat is strongest and the breast in the middle where it's weakest, which is the reverse of how a whole bird sits, and it drops the roast from 60-75 minutes to about 40-45. All the skin also ends up in one plane facing the heat, so it browns evenly instead of shielding the underside. What you give up is the presentation of an intact bird, and the cavity space you'd have used for aromatics.
Truss loosely or not at all. Pulling the legs tight against the body insulates the hip joint, which is the part of the bird that needs the most heat and takes the longest to get there — so a tight truss makes the breast wait even longer and overcook further. Trussing exists mainly to keep the bird compact and photogenic and to stop the wing tips burning. If you want the wings safe, tuck the tips behind the shoulders and leave the legs open.