He was there for only a brief moment before flying away. Avoid situations that put stress on your subject. If a bird is showing signs of stress, remove yourself as quickly as possible. Such was the case when I photographed the Killdeer shown above. Knowing this meant that a nest or chicks were nearby, I disengaged and left its immediate area.
Birds are fast.
Even small songbirds, when perched, will often make sudden and unexpected movements. Knowing a bird's feeding habits, breeding season and other behavioral characteristics means you'll be in the right place at the right time. You'll also gain a newfound appreciation for these wonderful creatures. Obviously, the longer the focal length the greater your chance of capturing frame-filling bird photos. For the best results, try to get something in at least the 300mm range.