Should we be concerned after finding two sets of dead baby birds over the last month? (One set were three small embryonic looking birds, and the second set found today were four baby birds ranging in size and development.) They have been spread out over multiple locations. We do have large trees, but this seems like a lot of birds to keep finding.

No, two sets isn't anything to worry about. Sad, but not concerning. Strong winds or storms will knock baby birds from their nests. Sometimes cowbird babies will push the host babies from the nest. Sometimes babies fall when they are shuffling around each other jockeying for position to get fed or as they are trying to leave the nest, etc. Long-story short, it's tough being a baby bird, and lots of them don't make it. If you start seeing a lot of adults birds that have died, then that's cause for concern.