The red rump, or indeed red anywhere at all would indicate a male bird. The leucistic patches might otherwise be red thereby making it an adult bird.
I don't think that finches remain juvenile for long. Some of the blush I noted on your video seemed to be transient, as if caused by a reflection from the glass superimposed over the the propagated image. The red on its rump was definite and real. Your honour, may I approach the bench?
"If the photographer in this case had captured a video of a leucistic finch, not through glass, but through leucite (perspex), would that not be leucitic evidence of a leucistic finch?"
"SS, this supposition is completely irrelevant!" shouted the judge, with stern disapproval. "One more non sequitur like this and I'll hold you in contempt of forum. Do I make myself clear?"
