Good to have an opinion of a kingfisher expert, thank you; as I was not aware that kingfishers would 'walk up' perches'... I assumed that there was a fine mesh over the 'hole' in the tent, but maybe not - and that the bowed perch was positioned that way to be above the heat mat.
@Andrew Swales Yes, they will hop along perches if they feel so inclined. I don't think there's any mesh over the center there at the moment, I assume that's what the rolled up section at the top is.
Something else to consider: this photo was taken on a very cold January day. Presumably the main function of the tent is to provide extra warmth (especially by being closed at night).