2x Extender or a 400mm F5.6?

I currently have the 70-200mm F4 and would like to shoot birds on the lake outside my house.
My question is, can I capture the birds with a 2x Extender or should I save my pennies and get the 400mm?

Save for the 400.There is a lot of past discussion about 400 5.6 vs 300 4 IS vs 100-400 IS.Search around the forum to find the pros and cons.

The 2X is knocking $400.00 new.

You can get the 400mm prime used for around $950.00

You "could" use the your Zoom with the 2X, you won;t be able to autofocus,. and the image quality will take a big hit.

So my advice would be to put the money towards the 400mm.

The 11th commandment... Never use an extender to achieve the focal length you need. If you need 400mm, then buy a 400mm.

I have a 2x extender on my 70-200 f2.8 II, and it does auto-focus, but it is slower than the lens by itself. It is also slightly soft on the IQ.
On the 70-200 f4 I don't believe it will Auto focus. You will be stuck with manual focus and that could be tricky on 400mm.

As others have advised, I say go with the 400mm or the 100-400mm. After I paid $450 for my Canon 2x III, I wish I had just paid the extra $1000 and got the 100-400. I may still do that and sell my extender.

I will say that I have had fun with the extender, but without AF, forget it.

I bet OP has no idea that he will lose AF with the 2x...

Only if the T.C. is a reporting T.C. The best option is almost always a longer lens!

the 400 5.6 is an infinitely better decision.

Thanks for all of the input. I did not realize the auto focus would not work with the extender.
I will definitely save up for the 400mm.