The NEX-6 won't be too sharp when adapting other lenses from other mounts - but will work fine with newer, longer Sony e-mount lenses. Whether it's worth buying one of those new lenses probably depends on how much use you'll get out of it. The 55-210mm lens is very basic, consumer-grade optics...it's fine for basic telephoto needs, but will definitely not do as well with details for something like backyard birding. You should see pretty solid improvements going to a newer generation lens and of course longer reach. Sigma's 100-400mm E-mount lens would give better image quality overall, and twice the reach. Another good option to consider would be Sony's 70-350mm G OSS - it's got a little less reach at the longest end, and a little more at the wider end, and is optically very good. Of course, the NEX6 body is a few generations back, but still has a good sensor - and as long as you don't need fast tracking focus abilities and aren't trying to post billboards, the results should still be plenty good. I shot with a NEX-5N and the 55-210mm lens for a few years and still have some favorite gallery shots from that combo that hold up today. And if you get one of the newer lenses and decide you're really enjoying the birding and photography, you could upgrade the body to something with a little more resolution and tracking focus - the A6000 and A6300 cameras that followed are both nice step ups with 24MP sensors and nowadays can be found used fairly cheap, so you don't even have to jump up to the newest models to see nice improvements.