Nikon SLR Fast Shutter speed too dark?

I recently Purchased a Nikon D3100 and I'm going to be doing some bald eagle watch this weekend. I would like to catch a clear crisp image of the birds weather they are in motion or somewhat still.
on sport mode of the camera I can select on avg a 1/25 to 1/500 s shutter speed depending on the lighting I guess, but if I go into my S mode(shutter priority) I can select what ever shutter speed I want although the higher my shutter speed it the less light my sensor can absorb and handle, I think that's the theory behind it I was curious to what application a 1/4000s shutter speed would be used? And if I want to take a 30 s observation on lets say a starry sky, it says its too dark and won't take a picture but I think i found a way around it by turning my auto focus off to bypass it?

I'm more concerned about my fast shutter speed having sufficient light, I played with my iso settings and +/- brightness to let in more light but it's not enough

many of you are probably thinking turn my flash on however I'm already outside its daylight out and I'd like to do rapid shots, 3 frames per second. Which I don't think the flash will do that

what's are good setting for my type of work I will be doing this weekend (birds)

shutter speed
ISO
exposure comp

If you are just watching birds, that are stationary, you could get away with a shutter speed as low as 1/250. You don't need 1/4000 shutter speed, that is ludicrously fast. You don't want to set the ISO too high, or you will see a lot of noise. The reason your camera won't let you take a picture of the night sky is because your camera can't auto-focus when it is that dark, it is not that it is too dark outside for a 30s shutter speed (that's a very long shutter speed), but it's saying "hey I can't focus on anything right now". When you put it in to manual focus, the camera is saying "OK, you got this." I suppose you could use a 1/4000 shutter speed in an insanely bright room for uber slow motion video (but not with your camera). Exposure compensation is simply an automatic system that takes your current exposure settings (shutter speed, aperture), and adjusts it by up to 3 "stops". For you, I would put your camera in the sport mode, and just let the camera decide what is best.

You are NOT using your light meter. It will tell you if you are over or underexposing your images.

Look on pages 79-82.

* Exposure compensation is a very advanced tool and unless you fully understand the fundamentals of photography, you may want to wait to use it until you are more experienced using your camera and can make informed decisions when it come to making slight changes in exposure based upon the reflection qualities of the subject (very low and highly reflective surfaces).
* ISO is the only option for increasing shutter speeds if your lenses are rather slow
* When using high ISO's, you will need some kind of noise reduction in post as well as the noise reduction available on your camera in its menu (page 134)
* Shooting at 1/4000th second may be fine for shooting humming birds, but usually 1/1000th second is fast enough for shooting larger birds
* Most wildlife photographers are disciplined shooters and never shoot in the continuous shooting mode, but time their shots based upon their knowledge of the animals behaviour in the wild

Shooting birds (or any animal at night) will require much more sophisticated light setups.

This would include lighting the nesting area (be careful here that you are not shooting protected animals without a special permit) and a blind where you can observe the animals with a night vision system

Usually, S mode is not used because using extremely fast or slow shutter speeds will easily lead to under or overexposed pictures because the camera is unable to compensate with aperture or ISO settings.

Instead, use A mode instead. To allow for the fastest shutter speed possible without underexposure, set the f/stop value to the lowest (this will let the most light in), then set the ISO higher if the shutter speed is still not high enough.

If you're shooting moving subjects, you may also want to change your autofocus settings as well. Try continuos autofocus mode with 3D tracking; this allows the camera to automatically "track" the subject as it moves around within the frame.