>> In the UK, I think we just call the activity you describe as 'Birding' and those who do it as 'Birders'. I've heard about it a fair bit and it all sounds jolly interesting.
Same here, but on this forum and all photo forums I am aware of, "bird photographers" refer to themselves as "birders" too, although their objectives and time sinks are totally different.
I'm sure I am not the only one here that travels in the "twilight zone" in between, and there likely are full fledged and very experienced "birders" here but I don't think it is the typical "birder" here.
You don't see the value in an 80-400 because, unless you are cutting barbed wire and trespassing on airports you are not likely to be overfilling your frame, especially if you are interested in serial numbers.
For a sports shooter it solves a huge problem because they may carry up to 4 cameras (maybe more?) ready to shoot, just to cover all the possibilities, from the far end zone and including that last great wide angle shot a split second before a wide receiver runs them over on the sidelines .
Conversely, shallow DOF and lens speed is critical too, so I wouldn't want to call it. Of course, the latest FX cameras make sheer lens speed less important than it used to be, or at least less critical.
But whoever this lens was truly designed for, it was not optimized for birding or probably aviation. It is sort of a "Jack of All Trades, Master of None, and Affordable by Few".
That may be overly harsh because a lens that did everything that a sports shooter would want, at those focal ranges, would be too big and bulky to handle along with the usually obligatory big fast glass. If you want that focal length, and a zoom, and even one stop more speed you end up with a 200-400.
>> I'd pay right up for a 36mp DX cam though.
I think that will inevitably happen, and will be the minimum encore to the latest DX bodies (like a D3300/D5300/D7200), but I think somewhere in there we will have increasingly diminishing returns, if they have not already arrived, at 24 mpx (in the real world of mostly hand held shooting). So be careful what you wish for .