Better beamers, are flash extenders to use on the front of the SB800. It's basically a fresnal lens attached to a mounting bracket to extend flash distance. It's really a must for 300MM+ focal lengths. Especially if the subject is some distance from the focal plane. You might want to also look at using a flash bracket such as the Wimberley Telephoto Combo #4 or the RRS 91B, and an SC-17 or SC-28 cord. This will take the flash off of the same axis as the lens and help to reduce steel eye.
You can get away without using the flash extender and brackets I'd say for the small birds as they're usually quite close anyway. I really suggest having the flash permanently mounted though and try it out. Having a flash brings out more contrast in the bird, reduces the flatness, might also help you with having more DOF aswell as you seem to be trying to use the most of ambient light, and if there is no sun it will add a nice catchlight in the birds eye.
Don't be afraid to use it, at first I never bothered and have since decided to use it. Stick the SB800 and use TTL or TTL-BL, don't put any defusers on (you can try if you're close to the bird). Then adjust the EV (up down arrows) compensation to -1.3. I usually shoot using -EV. Then just set your aperture and shutter speed as per normal.
If the in camera meter bar is at 0 then you're using available light to expose correctly and just using the flash to fill shadows. You can experiment with mixing ambient/flash, by adjusting shutter speed and aperture. For instance if you go narrower aperture to get more DOF you'll see the bar is in the negative. What will happen is in the flash prefires it will tell there isn't enough light to expose correctly and will so give more flash output then say at 0 on the metering bar. So this will start using the flash more as a source of main light then as fill.
Now I could be all wrong here but that's what I've found by experimenting. You might already know all this. However if you did, I can't see why you wouldn't be using your flash