Baseline camera settings for bird photography:
1. As a general rule, I use Manual mode and set the lowest possible Aperture, highest possible Shutter speed, and lowest possible ISO.
2. In good light, I use Shutter priority mode and set 1/1000 second to start. Auto ISO works well in good lighting conditions. The camera will figure out the Aperture for you.
3. In low or continuously changing light, I use Aperture priority mode and set it wide open (usually f5.6 in my setup). Auto ISO will allow the camera to figure out the highest Shutter speed based on the lighting conditions.
Note: High ISO values produce noisy images with less dynamic range. If Auto ISO is selecting values higher than 3200, I will turn off Auto and set the ISO values myself. I don't like going over ISO 800 if I can help it.