@james -- There's a nice intersection between what you're saying about photos vs. memory vs. live birds here, in feedersketches and also birds in flight and a discussion we're having about illustrations vs. photos vs. sketches over in LOL.
What I'm realizing is that sketching a live bird is sort of a composite drawing (like field guide illustrations are) and a memory drawing because birds move so quickly. To quote Zen of Seeing, the book that inspired our Drawing to See activity,
How to draw a crowd? It seems impossible to draw this chaos in which every single element, every individual, moves constantly. Perhaps the warp and woof of the composition is an instant vision, flashing in the mind the moment I begin.
The hand moves and keeps moving, jumping as in a dance; lines without stopping move from figure to figure. As one of the apparitions (real enough to himself!) moves out of field of vision, his after-image sometimes persists on the retina, so that he has not really vanished yet and can be completed in absentia. But more often heads are matches promiscuously with bodies already gone. One woman's arm becomes another's.