Really, really good glass is a key component of Harry's work... this is what a $13,000 600 mm lens can do in the hands of an experienced bird photographer. A good degree of what you are seeing is based upon state-of-the-art photographic tools, in the hands of an experienced shooter.
Another thing is based upon shooting opportunity. If you have access to wood ducks that is a key. If you have 50 days a year when you're out in the marsh or the woodlands shooting, then you have many opportunities.
For those who are just getting started, zoos and Wildlife refuges offer opportunity. Most Wildlife photographers recoil at the thought of shooting semi-tame animals, and there is actually a sort of Unwritten code that strongly discourages these images from being used unless they are clearly labeled as having been made under controlled conditions. But for the beginner or intermediate shooter the ability to photograph wood ducks on water at close range, like 10 to 20 feet, this is where zoos and wildlife parks and wildlife refuges offer opportunity. If for example you had a 300mm f/ 4 prime lens you could use it to good effect on Ducks at 15 to 20 feet.
For bird photography the 500mm and 600mm lengths are very popular.