Motion in the image makes a bird look more lifelike.
In normal light conditions you can quite easily freeze the movement of a flying bird with a modern digital camera. Yet I feel that an image of a flying bird is more natural looking when there is some movement, at least in the wings. Wildlife photographer Hannu Hautala maintains that birds become plastic when they are frozen in flight with a fast shutter speed. I try to completely freeze movement at times, but on the whole I prefer images that show movement and am therefore more likely to choose a slower shutter speed.
To get motion blur in the image, you need to consider the size of the bird and the rate of its wing beats when selecting a shutter speed. The larger ...