I will say in regards to the letter 'X'...
I recall I had a sketchbook when I was young which myself and my mother turned into a sticker book of birds. As a young child I was not big on taxonomy obviously, so the alphabet served itself a neat way to organise things.
So for each page there was a letter; and on that page birds that began with that letter. But sticker-books for realistic birds were hard to come by, so the amount of birds on the page were determined mainly by what was in the sticker books.
View attachment 660788
Some pages, like the Letter K had quite a lot of birds on them - with a kestrel making an appearance, as well as a kingfisher, knot, killdeer, kakapo ... but the 'I' and 'J' pages prior to later addition had just three birds each from what I recall.
And I did notice that many of the picture-birds were mainly from the British Isles or were easy to access in captivity; many of the 'international' birds were illustration; or sometimes my mother would draw a bird which lacked sticker, as we had a bird encyclopaedia she could use for reference.
And so it was with the letter X, which has, like many other pages, been lost for good. But if memory serves on that page was a
Xantus' Murrelet* - which was the only bird in the encyclopaedia that began with an X. I believe the 'U' page had a drawing of an umbrellabird for similar reason.
*Since the encyclopaedia's publishing ornithologists have determined that there are in fact two species of murrelet within the California Current system; Scripps' and Guadalupe. Though I still do call Guadalupe as Xantus' simply because it bears the original scientific name [
S. hypoleucus]