The imagery of nature
The bird represents a natural point of view, and it sees nature first and foremost. Yes, the bird notices the unusual humans, but from afar, and for the first portion of the flight, the bird barely notices that they are different. Over time, the imagery moves into the future, but still the heron notices natural landmarks, and the heron operates through nature, within nature, thinking about nature. The descriptions of nature are lovely.
The imagery of life and circles
There are many cycles in this story, pointing to the periodicity of time. Life is depicted in circles, because the day trip begins and ends on the ground, and the bird is moving according to instinctual migration patterns, and the bird describes the strangeness of his behavior to himself. He doesn't understand why he does any of the things he does, but partially, this is due to the fact that he only experiences himself in any given moment of time, not seeing the whole life cycle. That's what is hinted by the imagery of wheels and invention, which the bird sees from above.
The imagery of flight and transcendence
The bird is concerned with flight and navigation, but traveling in the air is so fast compared to the trudging he watches below. This imagery of flight might make the reader think of transcendence, or transcendentalism, because the imagery is of an earthly animal soaring high in the sky. The bird doesn't permanently live in the sky; he has to land and rest on the ground, so, to him, the flight is a journey that begins and ends with the ground.
Time and the progress of man
The bird sees through watchful airborne eye that as the day goes along, the progress of man becomes more and more apparent. The imagery involves lots of smoke, including pollution from industry, but also chimneys in houses that are progressively better and better. The imagery depicts human progress, and it represents the dual nature of time to both improve and deteriorate, because human progress is having an adverse effect on nature.
