I've discovered a couple of bird nests in our backyard. One I can't see at all but hear the babies. It is under the patio roof. The other is too high to see into the nest but I have seen little beaks poking just above the rim. One is a sparrow nest the other is a Robin nest. I've had fun watching the momma's and daddies feeding their young.
This is how I knew there were two birds not just one coming to the nest to feed!
If you look close you can see a baby bird's beak just above the nest and under Mom/Dad's beak.