I'm sorry to hear of this with Devon. The behavior you're seeing - arching - is called opisthonos and indicates central nervous system involvement in a disease process - genetic abnormalities, neoplasms, metabolic diseases, malnutrition, exposure to toxins, trauma; bacterial, viral, fungal, or parasitic infections. The two most common conditions are vitamin D3 deficiency and lead poisoning. Nutritional imbalances are a common cause of illness in our pet birds. What have your budgies' diet consisted of, please? Seeds should compose less than 20% of their diet. A diet of mainly seed and nuts has excessive fat, carbohydrates, and phosphorus; marginal protein; adequate vitamin E, and are deficient in amino acids, calcium, available phosphorus, sodium, manganese, zinc, iron, vitamins A, D3 (necessary for efficient absorption of calcium), K, and B12, riboflavin, pantothenic acid, choline, and available niacin. Ideally, a balanced pelleted diet such as can be found here: www.harrisonsbirdfoods.com or here: www.lafeber.com/pet-birds should be fed as well as hard boiled egg yolk, pancakes and cornbread, the tops of fresh greens, dairy products such as yogurt and cheese, fresh fruits such as apples, pears, melon, kiwi, and berries, vegetables such as broccoli, carrots, beets, asparagus, cabbage, sweet potato, and squash, and even tiny pieces of meat.