A whole chicken does not cook evenly because the breast and thighs are different muscles with different targets. The breast is ready at 74°C / 165°F, after which it dries out quickly; the thigh still feels firm at 74°C / 165°F because it contains more connective tissue, and its texture improves at a higher temperature. In addition, the breast is exposed while the thighs are tucked beneath the body, so heat reaches the 2 areas at different rates. The problem is not the cook; it is the bird’s structure.
The issue comes down to one bird having two different targets. The breast is lean and has fine muscle fibers; when it reaches the safe threshold of 74°C / 165°F, it is at its proper doneness, and a few °C more can make it stringy and dry. The thigh is a working muscle, and its connective tissue has not yet broken down at 74°C / 165°F, so the meat remains firm; the temperature needs to rise further for that tissue to turn into gelatin. In other words, the 2 parts need 2 different things in the same oven.
The second reason is the bird’s shape. The breast sits on the outside and higher up, where it meets the oven’s hot air directly. The thighs are tucked beneath and inside the body, and they also have thick bones; heat reaches them later and more slowly. As a result, the part on the outside finishes first, while the part on the inside finishes later. The larger the chicken, the bigger this difference becomes because the distance is greater, and the longer the breast waits, the more it dries out.
The most effective solution is to change the geometry. Cutting the chicken open along the backbone and flattening it brings the two parts onto the same level and greatly narrows the gap. A second option is to roast the chicken breast-side down for the first half hour, then turn it over; this protects the breast from direct heat at the start. A third option is not to roast it whole: cut it into pieces, put the thighs on the tray first, and add the breast a few minutes later. A fourth option is to place the thighs in front of the breast, on the side of the tray facing the oven’s back wall, where the air is always hotter.
There are also ways to manage the heat. Covering the breast with foil slows its cooking and gives the thighs time to catch up; remove the foil during the last twenty minutes to brown the skin. Rack position also changes the result: the oven’s lower rack gives the thighs more heat, while the upper rack gives more heat to the breast. Brining also helps because a breast held in salted water is more forgiving of overcooking and releases its juices later. Stuffing the chicken with lemon or onion does the opposite: the cavity heats slowly and extends the cooking time.
Make the decision with a thermometer, not by cooking time. Take two separate readings, in the thickest part of the breast and thigh, without touching bone in either of the 2 spots. The chicken can be removed from the oven when both the breast and the thigh read at least 74°C / 165°F; if the thigh has reached that temperature but is still firm, separate the thighs and cook them for a few more minutes. Color is no substitute for this measurement; pinkness near the bone alone does not mean the meat is raw. Let the chicken rest for 10–15 minutes after taking it out of the oven before carving. Its internal temperature rises by a few °C / several °F during resting, so it can be removed as soon as it reaches the target temperature.