Hollow tree

Today I went a walk at Dalzell Estate with my friends, Marion and Joan. We started off at Baron's Haugh RSPB reserve and walked part of the Clyde Walkway before returning on a circular route to the car park. On our walk we saw a great variety of birds, wild flowers and trees, including a lime walkway and a yew tree walkway. There were also many of these hollow trees that you could literally see through! Amazing that they are still supporting their branches. I wonder how old they are?