In my quest to get closer to some birds, especially waterfowl who are often quite distant, I spent $250.00 on a teleconverter. It attaches to the camera body and the lens, effectively doubling the focal length. The big disadvantage is manual focus and loss of two f stops.
I’ve taken a few shots, and I think I may keep it, despite the drawbacks. It’s probably only going to be useful in very good light.
I’ll keep experimenting.

These Ruddy Ducks were a long way away and in poor light, so it’s not too bad.
f13 1/500 ISO 560

This Mallard was also quite distant, and the shot lacks sharpness.
f13 1/320 ISO 800

A distant Bufflehead.
f13 1/500 ISO 560