Juncos and moss

On several occasions I have watched, through the window beside my computer, as Dark-eyed Juncos picked something to eat from the clumps of moss growing on the branches of a Vine Maple tree in my yard. It was happening again today. So far I have not been able to determine what they are finding there. Is it part of the moss itself, which is Orthotrichum lyelii, or is it something hidden (or hiding) in the moss? Maybe other observers can help me solve the mystery. Vine Maple is found only in the Pacific Northwest, but I don't know if the moss is found elsewhere.