BIF is really tough on many fronts. One element not yet addressed is exposure, especially when confronted with sudden opportunities like the one posed here. Birds have a diabolical propensity to shift background settings suddenly and frequently. Starting from a water surface with a forested backdrop, now they climb increasingly into partial and then full sky, sometimes blue but sometimes including white clouds, or worse, all white. I have increasingly tried to prepare in advance when shooting in 'birdland' by using manual exposure settings. Either a spot meter reading with test shots on resting birds (or objects of similar character nearby), and adjusting from there. At least the bird will be well exposed whatever they do. It is easy to quickly shift aperture this way or that if a tweak is needed during flight. For birds incoming, the issue is making them silhouettes in a bright sky. An alternative is use spot metering with exposure lock when in A priority, for example. That's useful in a jam when shooting in that mode anyway, and along comes a flock of birds. I have my Fn button set for spot metering for this very reason.