Firstly, try posting in the relevant
photography forum - was very useful for me.
I had written a reply assuming you had a Canon body, but then I looked at EXIF on photos on your flickr and saw you had a Nikon... So, slightly rewritten and considerably less relevant, here goes...
I found from researching the Canon range - and I assume Nikon to be similar - that once you get past 300mm the prices hike really, really fast and by 500mm were well out of reasonable range. I also found that for wildlife you can do perfectly well with less than 500mm.
I recently bought a second hand Canon 400mm f5.6. I had originally intended to buy the Sigma 150-500 (Canon fit) but after asking a few questions and reading a few reviews opted for a Canon 400mm instead.
One useful review even compared an image from a 400mm Canon lens (albeit not the exactly the same as I eventually bought) blown up in size to match the same shot taken on a Sigma at 500mm. The Canon image is superior.
Other sites seemed to confirm this. So, for similar money, there was a viable 400mm alternative that seemed to produce better images than the 500mm. Now I don't know if Nikon have a similar lens to Canon's 400 f5.6 or 100-400, but it might be worth looking around to see if they do.
And if they don't? Maybe opt for a good 300mm and a teleconverter.