The ideal height to photograph wildlife, especially birds not flying (perched, standing, walking, swimming, etc.) is most often when the camera is level (pitch) and the bird is properly framed.
Basically, this is the same level as the subject.
If the bird is on the ground and the ground is flat and void of visual obstructions, getting flat on the ground is a great option and a ground pod is a great support for this position.
If the bird is in or on the water, getting to their level immediately becomes more complicated. The embankments of most water bodies are raised at least somewhat over the water and that makes it hard to get down to bird-level from outside of the water. If possible, and you are OK with the risks involved, getting in the water can be a great way to get down to close to the ideal level. Still, the comfortable/safe height of the camera (and likely the tripod head) above the water usually leaves the bird at a still-lower elevation.
The next option is to get farther away. If the bird is near you, the camera will be angled downward more than if the bird is farther away. Of course, moving farther away means the bird is smaller in the frame. That is, unless a longer focal length is used.
Very long focal lengths are ideal for bird photography for a couple of reasons. The obvious reason is that they make the bird appear large in the frame from a less-frightening (mattering only to the bird usually) distance. The other reason coincides with one of the reasons for shooting from a level: to strongly blur the background.
Long focal lengths magnify the background blur, giving images a more-strongly blurred background that makes the subject stand out. Aside from the perspective making the bird look good, shooting from a lower position pushes background farther into the distance, farther outside of the depth of field and making your long focal length lens blur powers even more magical.
For this image capture, I was wearing chest waders and a Gore-Tex coat and sitting in the water up to my elbows (where the Gore-Tex jacket became an important part of the wardrobe). The temperature was in the 40s F (single digits C) on this day, so I had many layers on in addition. The tripod was positioned so that the apex was just above the water line and I was bent over to reach the viewfinder. Note that I'm not saying that a low shooting position is comfortable, especially after over 4 hours of not moving. But, what is comfort when making a good image is at stake?!
Being as low as I could go and using a long focal length (840mm) on a full frame body provided a great background blur right out of the camera. Of course, it is hard to take a bad picture of a subject as beautiful as a wood duck.