600mm and you want bigger....you cannot be serious !!!
I'm not likely to be getting anything longer in view of the cost - ~£1,500 for 800 mm and £2,200 for 1200 mm new, a bit less for 'pre-owned' but 600 mm is not overly big for bird photography.
Anyway I was out this morning but the camera stayed in the bag as the light was poor, the mist was down and it was raining. There was a lot of activity however, more than I've seen for a long time.
Saw: Dunnock, Mallard, Cormorant, Reed Bunting, Carrion Crow, Oystercatcher, Moorhen, Goosander, Wigeon, Robin, Blackbird, Great Tit, Blue Tit, Long Tailed Tit, Grey Heron, Black Headed Gull, Herring Gull, Coot, Woodpigeon, Canada Goose, Greylag Goose, Chaffinch, Thrush, Reed Bunting, Magpie, Moorhen, Tufted Duck, Thrush, Lapwing, Shoveler, Cetti's Warbler.
Heard: Bittern.
Unusually, didn't see: Any raptors, GWE, Little Owl.
Disappointed not to see: Kingfisher (been a few weeks now).
Quite a few GW and Little Egrets around before Xmas but numbers have gradually reduced to the one GWE which appears to also have moved on.
White Tailed Eagle was spotted a couple of weeks ago but not by me (or many others!)
....you cannot be serious !!!