Possible bird nest in my tree

I was just sitting here looking idly out the window and realized that it looks like there is a bird's nest up high in the tree outside my window. I'll need to keep an eye on it and make a block at the base of the tree so the cats can't climb up there. It's a tree the cats like to climb up just for fun and I don't want them to get up to the nest.

So far I have not seen any activity around it, but I also don't remember seeing it there yesterday. I'll just have to keep an eye on it and see if the birds are still building on it or of they decided to abandon it due to a realization on the level of cat activity in the area.