In the book, right before the Owl departed, he turned to Mrs Brisby, and said that he wished her well, and hoped her son's life could be saved. He then goes on to say "You see, I can understand your particular need, for I face a similar problem"
Mrs Brisby says "You? But you have no Moving Day."
The Owl says "I have lived in this tree, in this same hollow, for more years then anyone can remember. But now, when the wind blows hard in the winter and rocks the forest, I sit here in the dark, and from deep down in the bole, down near the roots, I hear a new sound. It is the sound of strands of wood creaking in the cold and snapping one by one. The limbs are falling; the tree is old, and it is dying. Yet I cannot bring myself, after so many years, to leave, to find a new home and move into it, perhaps to fight for it. I, too, have grown old. One of these days, one of these years, the tree will fall, and when it does, if I am still alive, I will fall with it.”