Sir David Attenborough says he's been "completely overwhelmed" by birthday greetings as he turned 100, in a worldwide outpouring of affection for the British wildlife broadcaster after decades of trailblazing work. After more than 70 years of film-making, Attenborough's instantly recognisable voice is synonymous with the story of nature. He is still at the vanguard of efforts to protect the environment and has produced some of his most influential work in recent years. In Britain, Attenborough's centenary  is being marked with a week of special broadcasts on the BBC, a live concert at the Royal Albert Hall, events at museums, nature walks and tree planting.  "I had rather thought that I would celebrate my 100th birthday quietly, but it seems that many of you have had other ideas," he said in an audio message released by the BBC on Friday.  "I’ve been completely overwhelmed by birthday greetings from preschool groups to care home residents and countless individuals and families of all ages." He…