Nesting Great Tits.

This is my first visit to this RSPB site because I was very interested to see if anyone else had experienced Great Tits nesting below ground level. I have used an old fountain as an ornament in the flower bed next to the kitchen French windows and was amazed to see a Great Tit taking rabbit fur into the small space between one of the base arches of the fountain, a large pebble that had left a small space for entry and the top level of the soil. The nesting space is probably 1'square but at least 3" below ground level and he was/is using various garden ornaments next to the nest to check the area before disappearing inside. The eggs have probably hatched by now as there is continuous to-ing and fro-ing with food from dawn to dusk. There is a large flowering cherry next to this garden which I presume the young will be taken to for safety in a week or so. When I realised the birds were settling in there I was sure the nest would be invaded by weasels, rats etc. and thought this was probably a very unwise decision by the Great Tits! Maybe they nested there last year as well and I have to admit if I hadn't noticed the rabbit fur I may have been totally unaware of what was happening. I would really appreciate comments from other RSPB members as to whether this is an unusual practice or happens all the time. I must say I think this web-site is fascinating and look forward to keeping up with the RSBP info and comments in the future.