Many of these bird species mentioned are entirely unknown to me and I’m not going to try to find all of them in books but I will at least guess that for many of them they are hard to identify because they are closely related, as in have not diverged much, in which case there is no mystery about it. Also mostly they are still almost the same in morphology and size (which the C. Sparrowhawk & B. Goshawk certainly are not). There will of course be variants in things like bill length among otherwise similar hummingbirds. Also I would guess that for many of them it is the young, female and non-breeding plumages that are similar. With often the breeding males (the ones with the signalling functions) are often very distinct. So these are mostly different situations to what I was asking. Besides, my question was not about birds that are difficult to identify (that is not very interesting) but about why these two species show the same plumage pattern. As in I am not at all asking about our human perception, although some may see that there could be an element of circular reasoning in that.
Philip
