With fast moving birds, I think it is best to eliminate the ambient and use flash to freeze the motion. Doesn't this turn the shot into an "Indoor shot" which I would be better using spot metering and TTL (not TTL-BAL)??
The TTL BL vs TTL would be an exposure issue - TTL BL being a reduced fill level (assuming significant ambient is present). Technically, we would think we could achieve the same exposure either way with the necessary flash compensation to bring BL back up (however no bets, sometimes BL automation sort of has a mind of its own).
Motion would be a speed issue, and I think it would be about reducing the ambient a couple of stops (instead of reducing the fill a couple of stops)... reduced ambient so that the ambient blur would be dim, no longer a visible factor. So you would seem to have the opposite goal as fill (and I agree that TTL mode sure couldn't hurt
). Reducing ambient does change your picture, the background becomes dark instead of lighted by ambient.
The SB-700 duration is (for example) 1/10000 second at 1/16 power level (manual page H-17). That would be a lot faster than the 1/200 second maximum shutter sync speed. Low power is faster, but the flash will have to be relatively close. Two flashes would be 2x more, or one stop more. And the ambient exposure would be a couple stops even less, at the 1/200 second shutter speed. The 1/200 second shutter is slower and more blur, but only of ambient, which is made dim and not really visible. The flash duration is 1/10000 second, and would appear as the significant illumination.
It would help if the feeder were in the shade, not so much ambient for the flash to overcome (at the lower power level to be fast).
If concerned with the speed, I'd suggest camera Manual mode with 1/200 second maximum shutter sync speed (for maximum attenuation of ambient). And an aperture to put the ambient a couple stops down. if that is bright sun, Sunny 16 probably prohibits much ISO. The flash could be TTL or manual, into that aperture. If you wanted TTL, you could compare test exposures of the feeder to judge TTL vs Manual flash level, i.e., TTL should be the same actual power level as a properly exposed manual flash (only point is, then you'd know what TTL was doing). Manual flash should work too, if the distances don't change (with very slightly less shutter lag due to no TTL preflash). Possibly the birds position could change though, from one side to the other ? That might be a big change, compared to the low power flash position? I'm really not much into birds.
Hummingbird wings are the fastest, and it is commonly said to need 1/25000 second to stop them, which is about 1/64 power speedlights. You might read some at
hummingbird photography - Google Search for the ideas. I don't know your goal, but if not hummingbirds, I bet 1/16 and 1/10000 second works pretty well.
