Unravelling the enigma of bird magnetoreception

Not unlike the technicolor rainbow that the mantis shrimp sees with its magic eyes. https://theoatmeal.com/comics/mantis_shrimp

They™ think the birds see magnetic field lines. There's a good PBS Space Time video about it:

https://www.youtube.com/watch?v=0A1ouV7iD8o

Can probably feel it (with whatever biomagnets they have) the same way we feel a repulsive force when trying push two like poles together. So imagine a wall of that force you walk along. It's not that large of a force. You can leap the invisible wall and find another parallel wall if you want but it's just more convenient to follow it till you find a nesting ground or food or whatever.

Yeah, the proposed mechanism with cryptochromes works in the bird's retina, but it's unknown if it affects the normal visual field or is separate. There's some cool visualizations of it here: http://www.ks.uiuc.edu/Research/cryptochrome/ (from this paper: https://pubmed.ncbi.nlm.nih.gov/20655831/)

Would you want to 'see' which way is up?

There's nothing to see pre 100 years ago. And with human things unless the bird is actually centimetres away there is also nothing they could see given the resolution they could perceive. Literally there is only a direction.

And they don't even have a north or south direction, if swapped they can't tell. So it's less of a sense than up/down.