I don't pretend to be an expert photographer, but I've generally been happy with my wildlife photos, mostly of birds...bigger birds such as owls, American White Pelicans, etc. I generally use my old Pentax K5 and my almost as old, Sigma 150-500. This equipment wouldn't be regarded either as top wildlife equipment or the latest technology, by a long shot.
But I'm used, to it, I know what the strengths and weaknesses are and can usually compensate for the weaknesses. I am a fair weather nature photographer, mostly I go out during the day, when there is a lot of sunlight. I'm conscious of good shutter speed, given the fact that I handhold and that a 150-500mm lens is a long , heavy lens. I don't shy away from using 800-1600 ISO, think the pix are very good and I never enlarge beyond 11 X 14.
I practice a lot with my equipment. Before I took pix of Owls and Pelicans...larger , slow moving birds, I parked myself out at a bird feeder, with a bench to sit down on and learned, through experience to take pix of fast moving small birds like Chickadees.
This experience in capturing the small and fast, helped me to take pix of the large and lumbering.
So there you go. My wildlife photos standards may not be up to others in this thread. I dunno.
But I'm happy with them, and so are the few people that have seen my 'work'.
Oh yeah, don't be afraid of 800-1600 ISO on a modern DSLR, set your F stop at around F 10...plus/minus, on something like my Sigma 150-500 @ 500mm. Keep your shutter speed over 1/1000th of a second, I like 1/1500th or faster , shoot in good sun light and try your darndest to focus on the eye of the wildlife. Use the Pentax shake reduction setting, when hand holding.
Practice, practice and practice. Then practice some more. I've found with me, learning and doing anything, the more I do something, eventually the better I seem to get. Maybe this is a rule of the thumb or something.