I put that out sometimes too. although I don't like it much because when it gets wet it's heavy and not much better than mud in my opinion.....
What we will be doing and there are many threads on here about it..... SAND.
It needs to be a few inches deep. we haven't had a way to transport yet so we bring home a few bags at a time. We just put it in a small area for them and they love to eat it and 'bathe' in it and scratch in it. As soon as we put our dump trailer on the road we will be getting some sand. It's cheaper by the 'ton' or whatever load size you need than by the bag, at least for a larger area it is. Which looks like yours is.
Hope that helps and your birds look greaT!