I know they have wanted to try and put in foster chicks with this pair and it is still on the table, I've asked about it already again this year.
TPC is right - they do keep discarding their eggs - they can't even get them to do an analysis on them and they have tried several times in the past. They also keep moving sites quite quickly, no doubt to see if they will have better luck elsewhere.
The last place they were at I believe didn't have a nestbox however maybe, maybe now that they've chosen a site with a nestbox with a cooperative and enthusiastic landlord...we may have better luck. IF..they decide to nest and at least we know where they are earlier in the game this time.
The good news Bev is that everyone is willing to make it happen if and when the opportunities arise! Keep your talons crossed for them!!