The word "caught" is an odd choice of words here. The speaker is using it in the sense that we'd say, "I caught a movie on Friday night"—in other words, that he saw something. But choosing the word "caught" makes it sound like he caught it unawares, or caught it in the act. Or it might make you think of "catching" a bird in a trap or a cage. What do you think?
[…] how he rung upon the rein of a wimpling wing (4)
The bird is able to harness the natural power of the wind the same way that a horseback rider harnesses the power of a horse. There's beauty and hidden danger here: the wind is incredibly powerful, but the bird is able to "rein" it in.
[…] shéer plód makes plough down sillion Shine […] (12-13)
The image of a plough that is so well worn that it shines might seem out of place at first. The first eight lines of the poem (the octet) are about the bird's control over a powerful natural element—the wind. And the plough might seem to come out of nowhere. But a plough is often used as a symbol of man's control over the natural world, since it's a farm tool that is used to dig up the dirt to prepare it for planting. So the image of the plough actually goes pretty well with all the images of the bird controlling the wind.