Sigma 120-400mm OS Vs Nikkor 80-400mm VR?

I have read very few reviews on both these lenses to come to a firm decision. I know the Nikkor is obviously better, but is it worth the price difference? I do birding, but no action or sports photography.

The right answer will depend on your budget, how much are you willing to spend.

As you mentioned the Nikkor is supperior, and in my opinion you should always get the best lens you can afford. Remember that lenses are an investment, you may change your camera in the future but you will want to keep the glass.

Considering that the Nikkor lens will last for many decades, it probably IS worth the added cost.

I have Nikkor lenses from the early 1970's which I still use on my F4, D300 and D3.

They are robust and do their job.

It's the quality of the glass that makes the difference. The body is a housing, the focus motor is just that but the glass - it makes he technical quality of photos what it is. Go for the Nikkor.

They're both decent birding lenses. The relatively show AF of the Nikon shouldn't be much of an issue since birding is almost always at infinity or very close.

You could also check the 50-500 or 150-500 sigma lenses, those are ones you see the most with the birder on a budget.