It's a a bit of a mystery, and it happened quite suddenly - I estimated 25 birds and 11 nests under construction just last Thursday. I suspect it was the lack of mud. The last few times I had visited, I noticed lots of birds, but very little progress on nest building. The only completed nest had already been taken over by House Sparrows last week. I didn't see any House Sparrows taking an interest in the almost finished nests yesterday.
The lack of rain and the reduced amount of disturbance from construction activity in the immediate vicinity have resulted in a mud shortage there. I wonder if, as a group, they just gave up on that site and moved on to somewhere more suitable.
BB