Thanks, Legolas
I don't think tanglefoot would work. These birds (there's a whole family) are drilling all over the trees. Yes, more than one bird and more than one tree involved. This isn't the first year we've had this problem. Our neighbour lost their apple tree and we lost our walnut tree. These birds definitely do kill the trees. They don't leave them alone until they're dead. Right now they're working hard on our elm tree. I just hope they don't discover our apple trees.
Cellist