The departure of Castro, 20, whittled the competition on the top-rated TV talent show down to scruffy alternative rocker David Cook, baby-faced crooner David Archuleta and last-girl-standing Syesha Mercado.

Castro had been advised to pack his suitcase by no-nonsense "Idol" judge Simon Cowell on Tuesday after bungling the lyrics to Bob Dylan's 1965 classic "Mr Tambourine Man" and showed little surprise as he was given the hook.

He smiled broadly and clapped as host Ryan Seacrest broke the news and laughed during his final performance, a version of Bob Marley's "I Shot the Sheriff."

The Texas-born Castro, who collected the fewest votes from viewers, was booted in the final minutes of the show after performances by pop rockers Maroon 5 and season four finalist Bo Bice.

He attributed his downfall to his inexperience, telling Seacrest that he had tried to pick songs he was familiar with but "even then I screwed it up."

Seacrest said nearly 51 million votes were cast on the show this week, a season high.

"American Idol" pits singers from across the country against one another in a months-long competition. Past winners such as Kelly Clarkson and Carrie Underwood have gone on to music stardom.

While "Idol" remains America's most-watched TV show, this year has seen its second straight season of declining ratings.

Variety has reported that Nielsen data showed "Idol" was averaging 28.9 million viewers on its Tuesday and Wednesday night broadcasts -- down 8 percent from 31.3 million one year ago at this point.

The show's winner, who is awarded a recording contract, will be announced during the live finale in May.

Reuters/Nielsen

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