First identify the 20 species in the photos. All of them are monochrome, because they are essentially black and white, generally with some shades of grey. However I have used a little leeway, by including a touch of dark brown and ignoring eyes with brighter colours. Remember that some species have one sex that is monochrome and one that is not, so if I chose to include a smew, it would have to be a drake as the duck has a red-brown head.
But there is a second part to the puzzle. There is a 21st monochrome species hidden in the first answers: its name has 20 letters taken from their names in the order shown.
For each of these 20 species write down the first letters of its English common name and of the first and second parts of its scientific names (the generic and specific names). Use Zootierliste to check the names if you need to. This will give you a set of 3 letters, or perhaps 4 if there are alternative common names (although if a letter is repeated, things are simplified). For example if you think Photo 1 shows a Smew, choose the letters S, M and A because its scientific name is Mergellus albellus. When you have most, if not all, of the correct letters, select one from each set to identify the mysterious 21st species.
I will be happy to look at PMs with possible answers, but I will not reveal any answers until New Years Day. So please do not post any solutions in this thread until after noon (UST/GMT) on Sunday January 1st, 2023.
Merry Christmas and Happy New Year to all ZooChatters.
I will have to try to work out what this animal might be.