It is usually easy to tell the sex of a budgerigar over six months old, mainly by the cere colours, but behaviours and head shape also help indicate sex.
A mature male's cere is usually light to dark blue, but can be purplish to pink in some particular colour mutations, such as Dark-eyed Clears, Danish Pieds (Recessive Pieds) and Inos, which usually display much rounder heads. Males are typically cheerful, extroverted, highly flirtatious, peacefully social, and very vocal.[
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Females' ceres are pinkish as immatures and switch from being beigish or whitish outside breeding condition into brown (often with a 'crusty' texture) in breeding condition and usually display flattened backs of heads (right above the nape region). Females are typically highly dominant and more socially intolerant.
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When females get older, their ceres tend to be brown usually, females are often more bossy and rude with their own gender, but with males they get along better; usually when budgies of different gender are put together, they tend to be more kind to each other, some will not even fight or peck at each other for their life time.