Its been 3 years since I got my first DSLR. I now own an T2I with an Tamrom 17-50mm F2.8 non-vc (thanks to the forum) and a 55-250mm. I also own an 975mm refractor APO triplet telescope that I use for Moon and planetary shoot. I could use the telescope for bird photography but its 19pounds and I have to carry a mount with 30lbs counterweight. not very practical.
I once own an 300mm F4L and I really miss that lens (like an idiot I sold it). I decided I need to go back to bird photography and get an longer range lens. I really don't like the 100-400mm push/pull design so It leaves me with little choice : Sigma 150-500mm. I understand a lot more about photography since I've been doing Astronomy photography so I think I can handle that big lens stepped down to F8 - F10 to get the result I need.