Birds are feathered, winged, two-legged, warm-blooded, egg-laying vertebrates. They also have a beak,  a four-chambered heart, and a lightweight but strong skeleton.

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Cockatoos in flight

The yellow-crested cockatoo (Cacatua sulphurea) has a retractile yellow crest. The birds in Hong Kong were introduced, some say by Hong Kong Governor Sir Mark Aitchison Young who released the Government House's entire bird collection - including Cockatoos - hours before surrendering Hong Kong to Japanese troops in December 1941.