Birds (many of them) have the ability to see UV (ultraviolet) light, which we humans cannot see.
The following graphic illustrates that by comparing our (rather limited) human spectral range to the larger one birds have, including UV (ultraviolet), based on them being tetrachromats (they see 4 colors: UV, Blue, Green, Red approx. 300-700nm), whereas we humans are trichromats (we only see 3 colors: Blue, Green, Red, approx. 400-700nm).
Be reminded: The pink-magenta UV "color" shown here has been chosen to make it better visible for us humans, it is a "false color", as per definition UV light (approx. 300-400nm) has no color!
So how may then birds look like, seen through birds eyes? Possibly like this (it is simulated bird vision on the right):
I have used this to make visible in still photography and videography what birds may seen of a bird protecting device, the FireFly bird diverter, which may bee seen HERE
But how has this been done? Well, in principle using photographic technology, but with a twist:
Now this type of multispectral photography needs a modified camera (internal filter stack removed and replaced by an UV and visible light transmitting, clear fused silica window), a suitable UV transmitting special quartz-fluorite lens (the UV-Nikkor 105mm being one of only a few ever made ones), and special filters which allow to record UV images (such as the Baader-U UV transmitting, but visible light blocking filter), as well as filters to record the normal visible light (Blue, Green, Red).
The resulting UV and VIS (Blue, Green, Red) images are then suitably combined into one "multispectral image", in our case here the "simulated bird vison" image. [That term "suitably" describes a rather complex mapping process which I will not describe in detail here. NASA uses similar processes to generate their amazing images from their Hubble space telescope or their Earth observation satellites]
[all graphics shown here are (C) Hammarprodukter AB and Dr Klaus Schmitt, used with permission]
If you are interested into that type of photography, HERE on my site is a lot about it, including my "cook book" style tutorial how to do it.
Stay tuned, more will follow on this fascinating subject...