Hi
@GordonM
Not sure anyone will be able to make informed comments without seeing the shots, knowing which metering mode you were using and what part of the image you were metering from. In general, no camera is going to be able to meter on its own with 100% or even 50% accuracy. With all other things being equal, a small, light subject on a much larger dark background will always lead the camera to overexpose the subject as its trying to make the background grey. In theory centre-weighted metering will go a little way to alleviate this but its never going to get it right all ( most?) of the time on its own and how well it manages really depends on so many other factors. I'm guessing from your description that a light bird on a large dark background falls into this category.
You need to use your judgement in compensation, and I doubt the R6 is any different from any other camera in this respect.
On the plus side: if they let you borrow it, you must have an honest face