Howcome my blue and gold macaw only talks?

when hes inside his cage and covered with a blanket at night?? hes going to be a year old next month, he knows how to say about 10 words, but he only speaks when no ones around him or at night when hes covered up? why does he do this? it also sounds like he says a sentence but i cant clearly understand what hes saying... can anyone help me please?