I told MWNN I'd call back and went over for a closer look. There, behind the fireguard, beak pressed against the spot where Killer Terrier's nose touched the mesh, was a wood pigeon - the sworn enemy, flying vermin that it's his mission to eradicate. Yet he didn't move. I think he just couldn't believe that the bird could have invaded his territory unseen and it was therefore a figment.
What to do? I shooed both dogs into the kitchen (Loony Shepherd was resting and non too pleased to be sent to her bed so early). I opened the doors through the dining room and conservatory and took one of the sofa throws and approached the fireguard. At the sight of the throw, the bird panicked, so I held it behind me while I slowly raised the fireguard. The bird didn't move. It was a young bird, by the looks of it, in beautiful condition, the russet ring on its neck glowing with health. I threw the throw over the bird and waited until it had stopped struggling. Then, hoping it hadn't died of shock, I took it into the garden, placed the throw on the lawn and unwrapped it. Apart from losing a few feathers, and a little dignity, it was fine and flew off, into the tree where it annoys our next-door neighbour, Aldo, by waking him up too early in the morning. (We've agreed to go halves on a shotgun.)
Killer Terrier's now on watch from his chair in the conservatory. He still hasn't worked out how the bird got in. Perhaps I ought to send him to chimney sweep classes and kill two birds with one stone the next time it needs cleaning?