How budgie cere sexing works
Male and female budgies produce different hormones, and those hormones change the colour of the cere. In healthy adults, males usually have a blue cere and females a brown or pale cere. The colours are clearest once a budgie reaches breeding age — around 4 months and older — so a young bird's cere is only a rough guide.
Age matters a lot
Baby budgies have different cere colours from adults and can be genuinely hard to sex. If your bird is under about 4 months, treat any result as a best guess and check again when it's older.
When cere colour can signal a health problem
A sudden change in cere colour can be about more than sex. The most important one: if an adult male's blue cere turns brown, that is not a sex change — it can be a sign of a serious issue such as a testicular tumour, and it needs an avian vet. In females, a very thick, crusty cere ("brown hypertrophy") is usually normal breeding condition, but if it overgrows and blocks the nostrils it should be checked. Any cere change paired with other symptoms — fluffed feathers, not eating, tiredness — is a reason to see a vet.
When cere colour isn't reliable
In certain colour mutations — albino, lutino, dark-eyed clear, recessive pied and similar — males keep a pink or purple cere for life and never turn blue. For these budgies, and any bird you need to be certain about, a simple DNA test (from a feather or a drop of blood) is the only definitive way to know the sex.