A couple of years ago, we had an adult blue tit and young laying dead on the ground below one of the bird boxes that we have. No signs of how the deaths occurred, so we contacted the RSPB for advice. They didn't really know the answer, but suggested that it might have been an house sparrow that had some how managed to have gotten into the box. In reality it still remains a mystery today.
One thing in our favour, that particular 'blue tit' box was used prior to the event and as been used afterwards. We now have blue tits in residence at this moment.
We tend to find that our garden pond attracts snails and mice, and due to the steep vertical banks, they don't survive to well. We have put a stepped piece of wood into the pond, and this might have been a lifesaver for some?.