William Byrd (/bɜːrd/; birth date variously given as c.1539/40 or 1543 – 4 July 1623, by the Julian calendar, 14 July 1623, by the Gregorian calendar) was an English composer of the Renaissance. He wrote in many of the forms current in England at the time, including various types of sacred and secular polyphony, keyboard (the so-called Virginalist school), and consort music. He produced sacred music for use in Anglican services, although he himself became a Roman Catholic in later life and wrote Catholic sacred music as well.
Thanks largely to the research of John Harley, knowledge of Byrd's biography has expanded in recent years. According to Harley, Thomas Byrd, the grandson of Richard Byrd of Ingatestone, Essex, probably moved to London in the 15th century. Thereafter succeeding generations of the family are described as gentlemen. William Byrd was born in London, the son of another Thomas Byrd about whom nothing further is known, and his wife, Margery. The specific year of Byrd's birth is uncertain. In his will, dated 15 November 1622, he describes himself as "in the 80th year of my age", suggesting a birthdate of 1542 or 1543. However a document dated 2 October 1598 written in his own hand states that he is "58 yeares or ther abouts", indicating an earlier birthdate of 1539 or 1540. Byrd had two brothers, Symond and John, who became London merchants, and four sisters, Alice, Barbara, Mary, and Martha.
There is no documentary evidence concerning Byrd's early musical training. His two brothers were choristers at St. Paul's Cathedral, and Byrd may have been a chorister there as well under Simon Westcote, although it is possible that he was a chorister with the Chapel Royal. A reference in the prefatory material to the Cantiones sacrae published by Byrd and Thomas Tallis in 1575 tends to confirm that Byrd was a pupil of Tallis in the Chapel Royal. According to Anthony Wood, Byrd was "bred up to musick under Tho. Tallis." Moreover, one of Byrd's earliest compositions was a collaboration with two Chapel Royal singing-men, John Sheppard and William Mundy, on a setting for four male voices of the psalm In exitu Israel for the procession to the font in Easter week. It was probably composed near the end of the reign of Queen Mary Tudor (1553–1558), who revived Sarum liturgical practices.