One of the vets talked me through Squeak’s blood work results today. Basically everything is normal except his calcium levels, which are definitely too low. So there’s no obvious reason why he should be not eating enough, though a calcium deficiency would cause lethargy. He eats junk food well enough, so he may have suddenly developed a distaste or an intolerance for his regular (healthy, vet-recommended) pelleted food. So we’re switching him to exclusively the high-fat version of the feed, plus giving him millet sprays to get his weight and appetite back up, and giving him cuttlebone to get his calcium up (normally caged cockatiels on pelleted diets don’t need cuttlebone, as they get plenty of calcium).
So we don’t know why Squeak has gone off his feed or is unable to get the nutrition he needs from it, but we’re going to try to treat it with the cockatiel of junk food to keep him from starving. Then we’ll check in a month to see if his calcium has improved.
So it’s going to be a long road for Squeak, unless something more obvious presents itself in the meantime.