I had the Bigma at one time because I felt it was a reasonably inexpensive way to get to 500mm for nature pics. And, it is. It's a pretty good lens and a great value for the money, but there are limitations.
The thing about it is that beyond a certain distance (not all that far..closer than you'd expect for a 500mm lens), and I don't know if badpickev has experienced this or not, beyond a certain distance, it seems that the lens doesn't resolve detail very well or things get fuzzy or hazy. This hazy or fuzziness, lack of sharpness, doesn't seem to be there in the Nikon primes or if the subject is close enough (inside that distance I'm referring to) when using the 50-500 Sigma. After you've had the lens a while you'll start seeing where that distance is where you get a fuzzy subject and be able to gauge it.
The Bigma can be used for in-flight birding with decent success but it takes time to get it's timing and it is a real challenge... it is a big lens and focusing is a challenge sometimes. Use of a tripod is a must IMO beyond 300mm with that lens and it's noticeably softer from about 420-500mm. It's a slower optic so I found myself shooting at ISO 250-400 most of the time, even in brighter daylight.
Sigma should update the lens, improve the optics a little and make it a 200-500 f/4-5.6 with a $1,500-2,000 price point. You never use this lens within the 200mm mark...after all, isn't the long end why you'd buy this lens? It's characteristics at 50-150mm aren't that good, but they'll do if it's either take a shot with the Bigma there or miss an opportunity. You'll never say, I need a 50mm-100mm today, let me take the Bigma.
If Sigma did that change, they'd have an excellent optic that would be very useful for nature photographers and they'd probably have the amateur and semi-pro market all to themselves.
If you are going to go with a slower optic, and looking for reach, I think a better option would be the 300 f/4 Nikon AF-S with a 1.4 or 1.7x teleconverter. It will be very sharp and even with the teleconverters the lens is still fantastic (staying within this relative price point). Beyond that there's the excellent 120-300 f/2.8 Sigma and with converters you can get pretty close.
Some of the Nikon 300mm prime f/2.8 lenses are out there used and can be had at relatively decent prices as well. Using converters on those is an excellent alternative.
Best of luck whichever way you go and please post some images!
Julio