@Vilen The 400 would be the mimimum I'd recomend, it will get you a lot closer than the 200mm, sigma make 150 to 600 in Sony mounts, still well under the 500mm, Canons 500 and 600 primes are both well up there in price but oneday the 600 prime wil be where I'm at, just not this year, gives me somethign to work towards for now. Heres a link to a review of a 600mm on a sony. One thing to worry about it weight, these long lenses weigh a ton, so if that will be a factor and you pay for agiant lens and then the weight restricts you from using it, then is there any point? You need to have toys that you are comportable to use to get the most out of it, otherwise its a lot of money to sit on a shelf. So something to consider. @ScottBourne uses the Olympus micro 4/3s for his birding shots now and they have equivelent lenses, being micro 4/3 they are a lot smaller and lighter so if your looking at a complete new kit worth a look. if you already have the Sony you'll get great shots with the 400mm but I'm guessing after a year and if you get right into birding you'll be looking for a longer lens down the track so you ay want to think hard on this one before you go and get a lens, if you cna borrow one for a weekend maybe have a try with the 400mm, my local camera shop here borrows lenses and if you buy the cost of the retal comes of the price, so somethig to consider before you fork out the hard earned dollars. Remeber even with a 600mm lens and relatively close to the subject you'll be cropping your image, for this image I was about 5 meters away (15 foot) from a relatively small bird, the full size image is 5472 x 3648 the image was cropped to 2966 x 1977. So even at 600mm I still crop into the image to get a decent sized subject in the final image. You'll soon learn you can never have a long enough lens, but need to play that off against weight and cost, if its to heavey you'll never use it and if its to expensive well you may go hungy for a while.