First, let me start by saying that most of the time the best bird images are taken from the bird's eye level. It makes for intimate images. It enables you to blur the background better as the background in farther in the distance than when shooting down at a bird catching the background right behind the bird. It gives you a perspective that most people don't see.

This means that I am mostly lying on the ground shooting, or should I say in the water and or mud or dirt. Equipment will NOT make you a better photographer yet it helps you to get better images as it makes photography easier and you more comfortable. Here are a few of my favorite non-camera or lens gear for bird photography:
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A ground-pod. A tripod can often not get low enough on the ground and even if it can, how do you move it inconspicuously while you crawl on the ground? How do you prevent the spread out legs from getting stuck in the ground, or on plants? Movement chase the birds away. I like to be able to just slide the ground-pod on the ground. They are fantastic. You can even make one yourself from a used cheap pan. The model I use was bought from Huga Nature in Poland. (I am not affiliated with them or get any benefit from promoting them)
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Wimberley Gimbal. Ball heads don't work well for bird photography as you constantly have to keep the gear up. Some will say that you can just increase the friction setting on a ball head to prevent having to constantly keep up the camera and lens but that comes at a cost of free and easy movement. Birds move fast so we need to be able to also move our gear fast, uninhibited. A Wimberley Gimbal carries the weight of the gear, it keeps it in the right position even when untouched. It re-balances the camera and lens automatically if correctly installed to prevent the front of the lens to drop into the sand when you let the camera go. You can move freely and they just work perfectly. I highly recommend this. There are many brands that sell these gimbals heads. I prefer the Wimberley brand. The one I use is the WH-200 Wimberley Head Version II (which I bought used). The ProMediaGear Katana Pro also consistently gets good reviews. (I am not affiliated with Wimberley or ProMediaGear)
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Blind. Blinds helps the birds to not see you allowing you closer. It also puts them at ease so that they slow down and act naturally. Besides, blinds make for good sunscreen. I use a blind that drapes over me so that I can crawl and move. Mine is from LensCoat. LensCoat also sells lens covers that are highly rated. (I am not affiliated with LensCoat)
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Lens cover. With an expensive lens crawling along the ground in and near brush, sand, and mud, necessitates me to protect my gear. Lens covers also cover the white lens so that it is not so flashy to disturb the birds or alert thieves. The one I use is from Huga Nature in Poland but LensCoat is also an esteemed brand for lens covers.
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The right clothing. Photographers often overlook the importance of what you are wearing. Being too cold or too warm will send you packing and end your photography. The other day I was sitting out in 16 degree F or -8.88 degree C weather for three hours to photograph Prairie Chickens (which never showed up). You will freeze without the right clothing. View clothing as you do your photography gear.