The title does a lot of the work here. An "earworm" is usually a song you can't shake, but Dave Bonta applies the word to something far gentler: the whistle of a bird, heard at the hour of the morning star, when the sky is still dark and the day hasn't really started.
The speaker seems quietly delighted rather than annoyed. Where an earworm is normally a nuisance, this one is a gift. The phrase "I shall have in my ear / all day" sounds almost like a promise, or a small act of keeping, as if the speaker is choosing to carry the sound along. The slightly formal "shall" adds a touch of calm certainty.
The poem is very short, and its spacing leaves room for silence, much like the pause before a bird calls at dawn. The mood is hushed, fresh, and content. A tiny moment of early-morning attention stretches into the whole day, suggesting how a brief encounter with nature can linger long after it ends.