How big a lens does it take to watch and shoot birds in Yalu River?

Generally speaking, at least you have to have a lens with a focal length of 300mm, and it would be better if you can have a lens with a focal length of 400mm, and if you have economic conditions, 500 or 600mm is beyond the expectations of ordinary people.

Many ordinary photographers use zoom lenses to shoot birds, such as Nikon's AFVR 80-400 mm f/4.5-5.6ED, which is economically impossible for the time being, and Nikon's AF-S VR 70-300mm f/4.5-5.6G IF-ED can also make do.

Shooting birds requires burning equipment. Without good equipment, it is difficult to play well. The Nikon AF-S VR 70-300mm f/4.5-5.6G IF-ED mentioned above generally shoots close-range forest birds, which is ok, but you said that shooting birds in Yalu River, without an 80-400mm one, I am afraid it will be difficult to enjoy it.