Hi Mikal,
He's not sure why it glows, only speculating it may have been used to attract a mate.
Quite fascinating, many thanks for sharing the link!
This really gets me thinking: As birds can actually see one well-defined colour in the wider UV band, I suspect saying that "birds have always known about the extra colours in the puffin bill" is simplifying things a bit too far.
What humans can see is that the bill is reflecting more light in the human-visible spectrum than "it ought to". It might be that to birds, the same applies to their extra colour, as part of the UV band is invisible to birds, too.
Basically, the Puffin's beak features the equivalent of a black-light paint colouration.
To translate the bird's experience into human terms, imagine the beak shining bright red if you point a blue spotlight at it.
I would think that's a conspicious visual effect, even if you can see the "incoming" light, and in my opinion, it's a fair analogy of how birds might perceive the Puffin beak's "extra colours". It's really more like "extra intensity", probably.
(Maybe fluorescent safety vests can be used as an illustration for this: They don't stand out because they're orange, they stand out because they're more intensely orange than they would be without being able to transpose UV light into orange light.)
Regards,
Henning