VATICAN CITY (Reuters) - Tens of thousands of people, many of them children, serenaded Pope Francis on his 81st birthday on Sunday, as the pontiff appealed for the release of Catholic nuns kidnapped last month in Nigeria. The crowd sang Happy Birthday in Italian as the pontiff appeared at the window of the Apostolic Palace overlooking St. Peter s Square for his weekly message and blessing. The first Latin American pope was born Jorge Mario Bergoglio on Dec. 17, 1936 in Buenos Aires. The former cardinal of the Argentine capital became leader of the world s 1.2 billion Roman Catholics in 2013.   In keeping with a long-standing tradition in Rome, the children faced the pope s window and held up small statues of the baby Jesus for him to bless. The statues are placed in home nativity scenes, a custom still practised in many Italian families. Francis, who has often warned against excessive materialism during the holiday season, departed from his address to say:  This is the real Christmas. If we remove Jesus, what is left? An empty feast.  He appealed for the release of six Roman Catholic nuns kidnapped from their convent in the town of Iguoriakhi, Nigeria, last month. Francis said he hoped they and all other victims of kidnappings could be home for Christmas. 