So, I have been keeping an eye on the bird nest on a daily basis since I found it and, suddenly, there are no birds! They were nowhere near fledging, as you can see from the picture at left that I took a few days ago. Today's picture at right is sadly empty. They must of been found by a predator, likely a racoon or a cat in this area.
As to what species they were, I'm not sure. They were fairly large chicks for how immature they were, yet the nest construction seems more consistent with some sort of small finch. Only once did I see an adult bird fly out of the tree, and I did not get a clear view of it. By size and color, it could potentially of been a Swainson's Thrush which are do inhabit this area, but in not particularly large or visible numbers. It could also have been a female cowbird which certainly rank right up there among my most hated of bird species. However, as I saw the bird fly off, its flight pattern seemed like that of a thrush of some sort. Also, as far as I know, cowbirds never take care of their own young, so it would be inconsistent for them to be feeding nestlings.
Whatever they were, we'll never know. But I'll certainly keep my eyes peeled for more easily accessible nests. We have quite a few nests in birdhouses in the garden, but it is much harder to get pictures without disturbed the birds a lot in those.