In general tail vertebrae of birds are fused to form which organ?

Generally Tail vertebrae in all the birds are fused to form pygostyle. Last three to four vertebrae fuses together to form pygostyle.

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Tail vertebrte in all the birds to form pygostyle and last three to four vertebras are joined to form pygostyle please kindly approved me

Tail vertebrae of birds are fused to form "Pygostyle".->Which helps in flying.->Last vertebrae are fused.->It also considered as a character which is modified for flying.