Bad year for bluebirds

Today I removed another empty bluebird nest. Two days ago it had a tiny mass of naked wriggling bluebird chicks a few hours old. Now there's no sign of chicks nor birds. No remains of any kind, which suggests a snake or similar varmint. This box was on a metal pole, but that's no guarantee against intruders, especially snakes. The other nest that had eggs is also empty, although the birds have started a new nest. I'm not terribly hopeful. The later in the season, the poorer the chances they will survive. Meanwhile, there are eggs in the box behind the house, but that's on a wooden post and besides it's much too close to the wrens (no, we don't keep wren houses, but they are prolific anyway) which means the odds are against success in this box too. That makes three nests with eggs or chicks destroyed by predators this spring - two by land and one by air. It's been a tough spring all around.