I agree with Susan; it looks like Ruby to me. I’ve seen her leave the nest for up to 5 – 7 minutes with chicks alone in the nest, and seen her leave briefly. It appears that when she leaves, it is to void or to cough up a casting away from the nest. Or, in one case, honor Buzz and/or visibly support him in defending the nest from an Osprey.
The bird in this photo is an adult, as told from the extensive barring on the flanks , the bright white body feathers, and the vibrant colors. The light, yellow eyes suggest a young adult. The three kids of last year, if they are alive, still look like juveniles. Yellow eyes, no red tail, heavy dark vertical streaks on the belly band, and little or no fine barring, all of which marks them as dumb kids, not a real threat to mate or nest. Their plumage is essentially washed out, duller brown than ever before because it has been bleached by sun and rain for 10 months. They are in “grunge” plumage, designed not to attract a mate, establish territory, whatever. Over the course of the next ten months or so, they will gradually molt most of their feathers (at least body; not necessarily all flight feathers) and acquire their “red badge of courage,” visibly signaling that they are at least theoretically ready to take or seek a mate. Many may not mate that first year of adulthood, though not necessarily from lack of trying. (Ahh, those college days....)
Ruby, however, is not going to look much better than she does now. (Buzz would never suggest that, however.) She is at peak breeding plumage, or at least was several weeks ago. This plumage will fade with continuing exposure to sun and rain, abrasion from all those kids seeking shelter, etc., and handling juicy squirrel carcasses in a crowded nest. She should begin replacing primaries while she is spending all her time on the nest, incubating and then brooding young. That is, she is not placing much stress on her feathers through extensive flying or requiring heavy insulation. Males typically begin molting primaries while their mate is incubating when relatively little is required of the male compared to now, with 4 or 5 beaks to fill. If demands are heavy, the molt can be delayed for several months (Not consciously, but by lack of hormone to stimulate the molt).
I am not aware of any reports that a nesting pair of Red-tailed Hawks would tolerate a third adult in the nest, with two documented exceptions where two females shared a nest and a mate. There have also been documented incidents where a single male maintained two mates and young on separate nests. (More to come on E!) The closely related Common Buzzard in Europe is also known to be polygynous on a regular but uncommon basis.
Two other notes on plumage. Look at how richly barred Ruby is on her flanks and legs. Finely and delicately barred, not coarsely streaked like a kid. This really serves her well as she incubates and broods because it provides effective camouflage as she spends 98% of her days sitting exposed on a nest fully exposed to everything that happens by, including people going into “Whole Paycheck” to buy poultry. The camouflage is important in a tree nest where the leaves have not yet emerged (which they generally do as the young are larger and more obvious), but particularly important on a cliff or commercial building nest where everything is out in public view all the time. (Talk about 24-hour news cycles on cable...) Buzz, however, is much whiter than his mate. That is great when he is defending his territory during the offseason, or seeking to attract a mate early in the year. Now, however, it is a liability that can only attract attention to his exposed, vulnerable mate and young. Rest assured, he is usually in the area with his mate in view or calling distance, but he is not a flag signaling predators where to find their next meal. As the chicks grow older, noisier, and more visible, and their wash in the nest and the remnants of their feeding frenzies accumulate, Buzz spends much more time visibly associated with the nest, such as sitting on the apex of the Atrium in full view of 30,000 commuters. At that point, the benefits of his visibility outweigh the risks. He is signaling every potential predator that they wont get to those young, tender squawkers without going through him.