The lens confusion!

Hi all.

I've been using the canon 100-400 L IS USM for a little over 8 months now. I've initially started off as a person enthusiastic about mammal photography only and hence got the 100-400. However, very soon, I found myself attracted to birds as well and now shoot birds every weekend (almost). I find the 400mm a little less than what I like when making images of small birds. I find myself wishing to buy a longer telephoto, which I might do eventually early next year. Had the canon 200-400 f4 L IS + 1.4x extender been announced officially, I'm sure there would have been no need for this post. the 400mm focal length, I find ideal for mammals and the 560mm focal length with the TC would have almost sufficed for the kind of bird images I shoot.

However, that not being the case and rumors running about that it might be as much as a year before that lens is announced (?), I started to think about other options, the first being the canon 300 f2.8 L IS USM II with both 1.4x TC and 2x TC, the other choice is the canon 400 f2.8 L IS USM II with a 1.4x TC, but the high price and the relatively high weight put me off. Then, there is the canon 500 f4 L IS USM II, which should be available from december. This seems to be the most tempting offer, but somewhere inside, I'm afraid that 500mm will be a little too much when shooting mammals in places like bandipur, ranthambore etc..Finally, there is the canon 400 f4 DO IS- the underdog. The focal length I am looking for and light weight seem attractive but talk about its low contrast rendering give me second thoughts. Also, the fact that it is a 1st generation IS lens and that 2nd generation lenses will be out in a month or so is a downside.

I will be grateful to anyone who can help me fix my mind about these glass.

Bhargava