Scott McKnight writes in “The Hum of Angels”:

I was visiting a bird-supplies store when I mentioned to the owner that my wife and I had owned a hummingbird feeder but had never once seen a hummer at the feeder, so we tossed it out. I concluded that there were no hummers near our home.

The shop owner asked where we lived, I told him, and then he said, "They are there. Not only do some of your neighbors have hummers on their feeders, but hummers are all over the village." What he said next was the take-home line: "You just have to have eyes to see them. Once you do, you will see them everywhere. They are small and fast and camouflaged, but they are not that hard to spot."

Eventually we bought a new feeder, filled it, and waited until our eyes got accustomed to the sight of hummers. We now see them everywhere. When other people go on a walk with us, we often observe a hummer—but it is rare that our friends spot one. It takes experience. You need to learn to spot them out of the side of your eyes and acclimate to their habits of zooming and darting and taking shelter on obscure branches and even on telephone lines. But once you've learned to spot a hummer you will see them everywhere because they are everywhere.

Like angels. They, too, are all around us. Few of us have seen one because we first have to learn what we are looking for. In a good book about angels, Martin Israel, quoting a friend, wrote this: "Eternity lies all round us and only a veil prevents us seeing it." The hum of angels surrounds us, and we only need ears to hear it or eyes to see them. Or perhaps a special sense for them. After all, the Bible tells us that Balaam's donkey could see an angel that Balaam himself could not see.