Juncos are common victims of nest-parasitism by Brown-headed Cowbirds. The cowbirds identify a junco nest with eggs in it, then the female cowbird lays an single egg in the nest and destroys the junco eggs already there. The juncos incubate the egg, and after the chick hatches, feed it in the nest until it fledges, then often (as this photo shows) continue feeding the cowbird fledgling outside the nest.

Here a large cowbird chick begs food from a smaller junco at our back yard feeder, see http://www.pbase.com/tgrey/image/99592428 . This was the first of several instances we've seen of cowbird nest-parasitism against juncos around our house.