Absolutely Frank, I agree - that is why I prefaced my post with "I think this is a case of situational requirements. " In case I was not clear, I meant that what each of us favours depends on our situation - which is completely in line with your approach.
Funnily enough, I often shoot in deep bush - so not the different from your expressed situation. The bird on the left of the two images I posted was in exactly that kind of environment - to get that image you will note both the shutter speed and ISO I used. There are so many variables in the environment, what settings we use, our camera body and the subjects themselves, but given we are being asked for our perspective on lenses in this situation, all any of us can do is express the pros and cons as we understand them within the context of the information we receive from the writers.
Forgive me if I am slightly bemused with your comment :"This is the main reason why I try to avoid giving recommendations about lenses for Birding or Wildlife photography,") after it seemed you did express your opinion: "For birding I find 100-400 most of the time adequate. So all depend on the needs. 800mm I think that it's too much for Birding. The maximum I would go would be 600mm" Perhaps I am misreading your intent. Either way, that is your experience and I am absolutely fine with that.
It think there is a consensus that teleconverters are one solution but something of a compromise, and the best results will come from getting an optic that is natively designed for the job. In wildlife that means reach, but how much reach again, is not a constant. The other constraint, always there is the cost and what one will carry.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
