Interpreting poetry is often tricky because the meaning is not always clear. But one interpretation that occurs to me is: "You should consider the possibility that every bird that flies experiences joys that are beyond your human understanding, since your human understanding is limited by your five senses, and those senses are insufficient to understand the joy of the bird." In the sentence, "but" means "that it is not a fact that". P.S. William Blake writes beautiful poetry, but it can be very difficult to understand it.

Why do you write “ev’ry” and “clos’d ?” These are wrong. It’s every and closed. And your sentence doesn’t make sense. How do you know how a bird cuts through the air. In an immense world of delight. Or it’s an immense world of delight. The last part doesn’t make any sense the way it’s written. You are referring to your five senses so the word “closed” doesn’t fit. And there is no meaning to the word “but”....who knows what you mean. I suspect you are quoting a piece of poetry but I may be wrong. What you have written is not good English that folks can readily understand in the real world without them asking “what are you saying?”