I guess they could climb on the dead tree trunk which is leaning on the big tree and get on the big tree from there. But whether they do it or not I can´t say, I didn´t see them there...
I think its deliberately been positioned like that to allow them access into the big tree if they want to. Climbing up the thick trunk of the living tree(oak?) might be next to impossible for them. But they aren't great climbers really (unless there's a food source up there), and might only do it occassionally.
