I think I may be able to help a little with your choice of the two lens mentioned above, having both in my collection, I know that trying to get that extra 'reach' with a Nikon set up is difficult, without spending a lot of money on prime lens.
The Sigma 150-500 and Nikon 80-400 are both heavy, are both a little slow in the AF department and their VR/IS is just acceptable ! these are just my very amature views .... BUT in good light they can both do a very good job .... here is the problem with both ! they need good light .... in basic terms I think they are two lens which need a serious 'make over' But still do the biz for the price, the 80-400 is so good to leave attached to your body all the time, as a walkaround camera it will surprise you with its versatility !
So a couple of comparison photos, these were both taken using a D700, but I hope the results will help ??
First the Sigma 150-500 (£799.00) photo taken late March, not best light, straight from camera, converted from RAW to JPEG only. notice serious vignetting (dark areas around all corners) almost always there ?? at 400mm 1/1000sec f10.

Second the Nikon 80-400 (£1179.90) photo taken today at about the same distance as the first, light fading ish at 17:30 once again straight from the camera, converted from RAW to JPEG only ... no vignetting ... therefore no need to allow for cropping ? and at 400mm 1/400sec f10.

So as you may have guessed I am coming out slightly in favour of the Nikon product ! but its a little more expensive, build quality is, in my opinion, a little better ! but you, as always, get what you pay for !!
I do have another suggestion for you ! but will await your views so far ! I hope this is of some help to you ??
Keith
