John C. has posted this excellent trip report on the forum on his birding trip to Cadiz
He notes on crested coots:
It soon became apparent that we had a problem with this coot; JC advised that a lot of the Crested Coots had been neck collared, so all we had to do was find a Coot in a collar and bingo!. No one seems to have noticed an obvious aspect of Crested coot physiology, however. After extensive field work it is apparent that Crested Coots are actually invisible under ultra-violet light, so in daylight, you simply cannot see them. All you see is a sort of shimmering in the reeds. That is why the clever Spaniards put collars on them: find the collar, find the crested coot. Sadly, evolution being what it is, the coots take off the collars during the day and hide them in the reed beds ( or possibly in the mud?). So the best time to look for Crested coot is a moonless dark night, when these all-black birds would be easy to see on account of the white triangular, square-topped frontal shields. As mentioned in the very beginning, JC had identified it as a bright, sunny night, so we had no cooting chance!