Roosting in the eaves of a nest box? Why would a bluetit do this?

My thoughts/guess are the same as PimperneBloke. And warmer air rises, so the air near to top of the box may be marginally warmer even if there was no camera.

Is it particularly cold where you are? I had a BT roosting in a bare camera nestbox but on the floor all winter long in relatively milder south east!

A further thought.

If the moss at the bottom is the remnants of old nesting material it could be harbouring hibernating parasites which have been 'reactivated' by the bird using it as a roost. Parasites will respond to heat stimuli.

The individual won't be worrying about incubation, its young and will therefore (arguably) put its own welfare first.

The phrase 'bird-brained' is a bit of a slur on birds (IMO). IIRC it has been seen that the neurons on their brains are more dense in their distriibution compared with mammalian brains. They're a lot smarter than they're given credit for.

Thanks PimperneBloke - that makes good sense. I suppose thinking about it, she's actually sitting next to the camera. What a smart animal! Should I try to make the box better-insulated? What temperature would be a good minimum to aim for?

Thanks tuwit - yep that makes good sense.

Thanks Wendy - No, it's not cold - we're in north Bristol. I was really surprised to find this bird roosting in the nest box, but it's been doing so every night since late September. Mostly, like your BT, she slept on the floor of the nestbox, but lately started roosting up next to the camera.

I cleared out the remains of the old nest and sterilised the box, then laid in clean moss ready for next year, but I think the bird does have parasites as I could see 3 or 4 of them moving over her feathers yesterday. Is there anything I can do to tackle the parasites? I've sterilised the box again today and there was no sign of parasites.

Totally agree about how smart these birds are!