the peculiar level of intelligence of the long-tailed tit is interesting - eg compared to a previous chaffinch.

the LTT is apparently failing to recognise itself in its reflection (and either sees a rival or, less likely, a soul mate) but it has evidently learned that attacking the glass isn't the way to go, unlike the chaffinch who used to attack all the windows at my parents' house.

instead it does keep zipping around the side of the house to look for a way to get into the room another way.

what would happen if I did have windows on that side of the house - especially since the LTT is already ignoring the window facing the same way as that wall (into the kitchen from which I'm mostly watching its antics).

it's possible the reflections aren't as convincing from that angle when interacting with the daylight.

now been outside to check and thinks the reflection is easily as strong in that direction.

it can't simplistically be that it's scared of my presence because I have also watched it from the room in which it believes the other LTT to be. (And a lot of the wildlife isn't particularly scared of me anyway.)

you should install said window for research purposes.

actually had the 2 small windows that side removed/filled many years ago (when having the double glazing installed) because they were completely occluded by the neighbours' evil leylandii hedge and hence purely a liability for thermal regulation.