I am going to use some time I have on a layover in the airport to write a little about the camera equipment I use. I am doing this mainly as a self-serving exercise as this is the question (one of the few) that I get most frequently on my Instagram page. I will be directing folks here for that purpose, so if you found this here from that avenue...welcome!
Before I talk about the specifics of the equipment I use, I want to discuss the balance of equipment, skill, and knowledge in acquiring good photos.
I will start by saying that equipment is important. Those who have good equipment seem to like to negate the importance of that equipment. They will say things like "an expert can use any camera to make great images" or that "the camera isn't important, it's the person holding it" and while those are or can be true in many instances, there are also instances where a better camera will without a doubt produce higher quality images. Many of those instances occur in the style of photography I like to do...low light hunting photos and images of fast-moving birds coming directly at the camera with objects in the background. High ISO images are a challenge for many less expensive cameras, and incoming birds are a challenge for many camera focus systems.
After I have discussed with several other photographers that do similar things to what I like to do, I like to think of camera equipment in the following way. If you gave me an entry-level camera and sent me into the marsh to take photos, that sure, I'd come back with some good images, but it wouldn't be the same as if I had my favorite setup. And why would I buy an expensive camera if it truly did not make any difference? And likewise, if you gave someone who had little experience my camera and sent them into the marsh, they probably would come back with some good images, but probably not the same as if someone who knew what they were doing was there. So, equipment is important in the fringes of performance, but for general photography, it probably does not matter nearly as much as does artistic ability or knowing how to use a camera.
With that above stuff out of the way, I will get into what I currently use for photographic equipment. I like to divide my photography into two parts...birds and not birds. Each has a different set of skill and equipment requirements. If you are here and reading this, you are probably looking for what I use for bird photography, so I will just cover that.
A couple of years ago, I made a switch from Canon to Sony gear. I have been happy with the switch. I've always wanted to have some version of a dedicated sports camera for bird photos because of the autofocus system involved in those bodies. They are made for tracking and predicting fast/erratic motion. I believe that taking photos of wild ducks in a marsh setting is probably one of the toughest tests for a camera's autofocus system that you can find. Keeping track of a one-pound Teal as it zigs and zags its way to the water at 40mph with a cluttered background behind it is virtually impossible for many cameras, and that is why I go with the sports cameras. I currently use a Sony A9 because it has a great autofocus tracking system. I will offer a bit of info on other brands and what I'd currently recommend in 2021 for bird-specific photos.
Sony - A9. I have used A7III and it's OK for birds, but not great.
Canon - The R5/R6 seems to be great, and the 1DX series are all great options. I shot with a Canon 1DX for a few years, and it was awesome. The newer versions of that camera are excellent, I am sure. I would not hesitate to recommend one of these cameras even though they seem old. The 7D Mark 2 is now available used for great prices and it's a very capable wildlife camera. The Canon 1D4 is also a great option that you would be able to find at a great price. The Canon 5D4 has a great autofocus system and is just great all around. I believe that the Canon 90D has a solid autofocus system but I have not used it so I cannot say for sure.
Nikon - I have not used Nikon cameras much myself but the D500 is an awesome bird camera. The D850 is supposed to be good as well.
Beyond those recommendations, I do not have much to recommend simply because I suspect that most other bodies would be disappointing...not to say that others would not work but they wouldn't be up to the level of these mentioned above. Get what you can afford and start using it and learning, and you will see if you enjoy it, and you will also see what skill and equipment shortcomings you are running into. If you are going to ask about any other camera beyond these suggestions, my standard response would be, “It could work”.
For lenses, I would suggest finding as long of a telephoto as you can get/afford. For years, I used a Canon 1 series with a 400 5.6 lens. It was a great, lightweight setup and many of my favorite photos are from that setup. Then I picked up the 100-400 v2 lens and that was great to have a little more of an option to zoom out if needed (although 95% of the time you will find that you are using a zoom lens for wildlife at its longest length). One of the reasons I got the A9 is that Sony has an amazing wildlife zoom that is not TOO expensive. It's a 200-600 and paired with that body it's a great, sharp, fast, accurate, and lightweight combo.
A note on lenses...do not expect a 600mm lens to solve problems of your photos not being clear enough when you "zoom in" on them. There is no way to make up for being too far away from a subject. If I want a nice, crisp, sharp photo of a single duck coming in that I could put on the wall, that bird probably needs to be under 30 yards and 20 yards is way better. Go out and measure off 20-30 yards sometime and see how close that really is. You can’t compensate for being too far away by buying better lenses or better cameras or using different editing software.
I hope this helps people understand what goes into some of these photos. The equipment is important, but it isn't the only answer. It won’t magically transform someone into an amazing photographer, although it will undoubtedly help once they have the skills and understanding that experience brings. These photos are HARD to get. Do not be frustrated if your photos don’t look like something you would see in a magazine right away. It will take time, research, failure, learning, experimenting, and work to get it all dialed in. I’ve been doing this for 12+ years consistently, sometimes 30-45 days a year. It took several years with minimal success until things clicked, and I was producing SOME images that I really liked.