Is a 5DIII worth getting for Birds in Flight?

I have a 1DMKIV now and I've heard the 5DIII handles noise a bit better, plus its a lot lighter when used with my favorite 100-400V2 birding lens.
Not sure of the AF as far as being as fast as the 1DMKIV.

I have both cameras and would not say AF on 5dIII is worse than 1dIV. It sticks to the birds pretty well. BTW for birds I chose 5dIII over 1dIV.

The 5D3 AF is very close to the performance you would have seen on the 1DX. It is very good, good enough for BIF and sports. The 1DX could drive the AF faster and was more surefooted, but the 5D3 sported the first major AF upgrade in the FF model line for Canon outside the 1D series.

It somewhat depends on how you set it up. If you're using multi-point AF, I would expect the newer cameras to perform a bit better due to improved processing power. On single-point, there is probably not a huge difference; I have a pro friend who shoots lots of sports action with a 1D mk II and single point, and it's about as good as my 1Dx or 5D3 under the same conditions. If you're in a dark environment, again the new bodies would lock on better.

In fact, I find the 1Dx, 5D3, and 7D2 all a bit skittish when targeting people with multi-point AF, until you lower the sensitivity a bit. If you're using multi-point for BIF, they would probably be a lot more responsive than the older bodies. I know the brief BIF test I did with my 7D2 and "all points" was phenomenal.