A touch question to answer... first thing I would say would be to ask what kind of bird shots are you trying for, in flight, sitting still?
At F6.7 it is going to be tough to get a focus lock at any time unless the bird is in bright lighting situations, if you have that it it best to try manual focus anyway. I had a Sigma 70-100 4.5-6.7 and it hunted so much in less than bright light I took very few successful shots, and the ISO had to be at least 100 to get any definition, and the depth of field was poor.
I have worked out a system for long lenses and focus that works for the most part. I now have a Bigma 50-500, so when I do get the shot it is a lot better than before.
1. You need a well lit subject, or a long external flash and higher ISO, and the smallest F# as possible for the best DOF.
2. A bright flash light (torch) to get the subject bright enough to focus lock. I have a Vistalight bicycle halogen light that is good up to 20m and has a push button on off, this gives me enough to get a focus lock.
3. Try to avoid in flight or fast moving shots except on a bright day, hopeless in my mind as the FOV and DOF is against you.
4. Use a monopod, tripod is too slow unless the bird is stuffed and unmoving. I have a Manfrotto monopod,with a ball head on it which is excellent for the Bigma, as it allows free movement of the lens while steadying it.
5. Take lots of shots, bracketing if possible... and be patient, it is really hard to get it right as long lenses are slow to focus due to the loss of light compared to shorter lower F#'s glass.
Here are a couple where I got lucky
With the 70-300 Sigma, in the dark using the bike light for focus, and the AF360-FGZ flash.
Another with the 70-300 but as you can see in very bright late afternoon light, also at full 300mm and 6.7 aperture.
And in bright light using the Bigma, at 10m (33') the Exif info should be there on the right click, properties.
Good luck