Nowhere in canon does is say that Hogwarts Acceptance Letters are delivered by owl or that they arrive on one’s birthday. In fact, the first chapter of Philosopher’s Stone indicates that they come in the post. Only when that doesn’t work does whoever is sending the letters resort to magical means. And never does it suggest that those means include owls. And Harry’s first letter comes a week before his birthday. It’s only a coincidence that Hagrid finally makes the delivery on Harry’s eleventh birthday.