Re birds landing in mid pond and drowning.
I have watched young sparrows land on vegitation out from the shore of my pond, they keep 'flying' until the footing underneath them feels solid. Only one of those I watched found firm footing and stopped flying, the other just took off again.
I think it more likely that yours suddenly slipped in off something and was unable to take off again. If the pond has steep sides with a couple of inches of freeboard drowned birds are quite common here. That is why my pond has a semi submerged shoaling shoreline, the following is a "during construction" photo
http://mysite.wanadoo-members.co.uk/...5-picture2.jpg
to date, 3 and a bit years, I have not had a bird or animal drown in this pond and it is heavily used by birds.