The little bird behaves in a completely natural manner when he's sure that no one's looking at him. He acts just like any normal bird would under the circumstances, biting a worm in half before gobbling him down and then drinking dew from the grass. He even hops sideways to the wall to let a beetle pass.
Everything about the bird's behavior is completely natural. He looks every inch the wild animal, with his beady little eyes displaying the caution one would expect from a creature prone to nature's predators. The way that he constantly jerks his little head about indicates that he feels himself to be in perpetual danger. This is the natural instinct that most animals have, especially wild animals.
This explains why the bird ignores the speaker's generous offer of a morsel of bread and quickly flies off. His life consists of looking out for signs of danger, and to the bird, the speaker represents a clear and present threat. Though the speaker means the bird no harm, the bird doesn't know that. His instinct is to take flight whenever he perceives himself to be in any kind of danger. In that sense, his flying away after the speaker offers him food is perfectly consistent with his behavior before the speaker arrived on the scene. Then as now, self-preservation is all important.