Buy a gram scale. Best 15 dollars you will spend on a bird.

Birds are prey animals and they're extremely good at acting fine. In the wild, looking sick gets you eaten, so the instinct is to hide it right up until they physically can't anymore. By the time a bird is fluffed up on the floor of the cage with its tail bobbing on every breath, that's not early illness. That's very late, and it's an emergency. Weight is what gives it away first, and it moves days or weeks before anything else looks wrong. You can't see it under feathers and you won't feel it picking them up. So: a kitchen gram scale with a flat top or a perch, weigh in the morning before they eat, write the number down. Same time, same conditions, every time. Once you've got two weeks of normal, a change jumps out immediately. The scale matters because a few grams is a lot at this size. A cockatiel around 100 grams losing 10 percent has lost 10 grams, and there's no version of you catching that by eye. While we're here, the diet half of the same problem. All seed diets are still incredibly common and the damage is slow rather than dramatic. Seeds run high in fat and low in a lot of what a parrot needs, and vitamin A deficiency is the classic result. It shows up as crusty or swollen nares, white plaques inside the mouth, bad feather quality, and a bird that gets one sinus or respiratory infection after another. Sunflower seeds and peanuts are the worst of it because birds pick those out first and fill up on them. Most avian vets want pellets as the base with vegetables and a smaller amount of seed on top. Converting a committed seed eater is slow and you do it while watching the scale, which is another reason to own one. Don't just swap the bowl and hope. Birds will genuinely starve rather than eat something they don't recognize as food.