Photographing birds in low light is a serious challenge. You need a long lens, and that means a relatively small aperture. Depth of field is important to get the beak and the body of the bird in focus, and that means an even smaller aperture. If you are using a teleconveter, you will lose either one or two f/stops, and in deep shade or when the sun is below the horizon, the shutter speed will be much too slow to expect sharp pictures even when using a tripod.
The only solution is to raise the ISO until the shutter speed is fast enough for a sharp picture. When using a tripod, how fast is that? A lot of factors come into play -- such as how long the lens is, how stable the tripod is, if you have raised the center column (never a good idea), and if there is any movement in the bird. I took this picture of a chestnut mandibled toucan in Costa Rica with a shutter speed of 1/320 using a 500mm lens, and it's tack sharp. My own rule is that when using a tripod, the shutter speed should be no slower than half the focal length -- i.e. in this case, I wouldn't go below 1/250 to be on the safe side. I used 3200 ISO, which I don't like, but I can remove the noise in post-processing. That is the compromise you have to make for a fast shutter in low light circumstances when photographing birds.