Happy Birthday is performed outside Buckingham Palace during the Changing of the Guard ceremony.
A new photograph of the King has been released as he celebrates his 74th birthday and embraces a role formerly carried out by the Duke of Edinburgh.
The King's new role was announced on his first birthday as monarch and 70 years after his father the Duke of Edinburgh was appointed to the post.
The role of Ranger offers oversight and guidance to the deputy ranger and his team in the day-to-day stewardship of one of the country's oldest landed estates.
Paul Sedgwick, The Crown Estate's managing director, rural and deputy ranger of Windsor Great Park, said: "We are honoured to have His Majesty as Ranger of Windsor Great Park, continuing a long tradition of the Sovereign and members of the Royal Family holding this role.
"Windsor has a wonderful heritage with many precious natural habitats.
"His Majesty's passion and commitment to the natural world will be invaluable as we seek to become a centre of excellence for environmental best practice, preserving and enhancing the Great Park for generations to come."
The King's 74th anniversary was marked by the Changing of the Guard ceremony at Buckingham Palace on Monday.
The Band of the Household Cavalry performed Happy Birthday during the ceremony.
Later in the day gun salutes will be fired across London.
41 guns of the King's Troop Royal Horse Artillery will be shot from midday at London's Green Park, then an hour later the Honourable Artillery Company will fire a 62-gun salute at the Tower of London.
Charles spent the day before his 74th birthday leading the nation in honouring its war dead during the annual Remembrance Sunday service at the Cenotaph.
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