I know that photograppy is about the user and not the camera but when I have been taking pictures of birds in flight my camera can't hold a focus point. I have it set up for back button focusing in Aperture Priority set to AF-C. I have tried the Dynamic, 3D and Auto focus modes to no avail. The focus light indicates a focus lock so I fire the shutter but when I look back in Capture NXD there is no focus point. Should I stop using back button focusing? My reason for saying this is that when I hold down the back button it will try to adjust the focus constantly. Is this why the shots aren't so good?
It's always tricky taking pictures of birds in flight - especially when your lens takes a few seconds to cycle through the focus range.
Here's what I do - Firstly, I use single autofocus rather than continual. If you're running continuous AF, then as soon as the bird isn't on an active focus point, then your AF is going to start hunting and is probably going to be way off when you get the bird back onto a focus point. At least with single AF, if you've managed to focus, then hopefully the range isn't going to change much.
Secondly, pre focus on a bit of the background at around the right distance. with single AF, that's just partially pressing the shutter while pointing in the right direction. What this does is get you almost in focus when you aim at the bird - so the AF can home in quickly instead of cycling through the full range (which usually blurs things enough that you can't see the bird part of the time).
And I usually use a single active AF point - that way, I don't have to worry about the camera deciding to focus on the wrong thing.
Continual AF works best for things that are moving towards or away from you that you can manage to keep on the focus points; if you can't manage that - especially with a lens that takes a long time to cycle through the full focus range - then you will have problems when the subject wanders off the focus points.
I am not a bird photographer but every year early in January we go out to shoot (figuratively) the American Bald Eagle return to The Columbia River Gorge.
I shoot using several very old but very high quality manual focus prime lenses in the 50-120 mm range. This is about the longest realistic focal length for this type of hand held shooting.
With most lenses out there, correct focus for birds in flight will be with the lens set at (a distance of) infinity. That's how I set my camera. When tracking an eagle in flight I press the shutter button when I get the focus confirmation light. Of course it's highly likely that the eagle will be in focus at this setting with or without the confirmation.
For any type of action shooting you have to expect to miss focus now and then. But this technique has given me a high rate of success over the years. Most of my off-focus images were the result of a too slow shutter speed and not the lens setting.
Of course with these lenses I am doing a lot of cropping so a high MP camera like my D810 is very helpful. Not familiar with how a D3300 performs since that's a fairly entry level DSLR. Without having actually used a D3300 I'm guessing that it's (the camera or your lens) focusing speed may be too slow for your application. Bear in mind that cameras are mindless machines. It will try to focus on any contrasting target you aim it at, not necessarily the subject you are trying to focus on.