The bluebird is exquisitely captured and this is better than the pictures in the handbooks people buy to spot birds with--that is a legit market. Just as pictures of people fall into many categories (medical illustrations, advertising, modeling, portraits, movies, sports, journalism, tourism--and there are different categories of portraits), so do pictures of animals. To be honest, in all my decades I never took a picture of a bird, and when I started coming here I was surprised how many do. I did not take pictures of cats, either, until I got one. My cat pictures fall either into traditional portrait styles, or into candid, existing light photography; I don't need long tele lenses or anything else special.
I don't think Adams photographed birds, either. Might we ask why?
Adams said (long ago) that it had been a very long time since the quality of photographs depended on the quality of lenses, and others here seem to agree in principle (as I do). But the types of photograph make different demands. I am not sure a proper portrait can be taken of a bird, because a first rate portrait reveals something of personality, character, or emotions. I think there is something tongue-in-cheek about formal portraits of dogs, cats, horses, because--well, they aren't really people, are they? If a bird does something newsworthy, like pulling the President's hair, that is one type of picture.
But I think digital photography has affected style and taste in photography. Overcooking can be a flaw, but it is also perhaps an essential feature of photography today that it look more like Oz than Kansas. That kept me away from digital for a long time. Now the public is getting used to it, or expects it. It is not merely a question of whether to look natural or real--that was always a question that was both relative and subjective. Much more color or much more resolution, or contrast, etc., may enhance some artistic efforts, and detract from others. But it was no long ago that some people put gauze over the lens, or Vaseline, for their artistic purposes. One even touched it up with sandpaper.
Some digital cameras let you shoot square format, which is a promising idea for many animals (such as birds)--good enough for Hasselblad and Rolleiflex. It changes the whole dynamic.
Going back over bird shots I have taken, the best ... (