Brute beauty and valour and act, oh, air, pride, plume, here Buckle! […] (9-10)
The speaker lists the qualities that the bird brings together ("buckle"). That word "buckle" also suggests that those qualities almost collapse into each other (as in, "my knees buckled under the weight of my backpack and I fell down the stairs"). The fact that the speaker uses the verb act instead of the noun action might suggest the movement of the bird. The speaker is most interested, after all, in what the bird is doing, so using a verb makes sense. But the speaker also wants us make us sit up and pay more attention to familiar, everyday things (like a bird in flight), so using familiar words in unfamiliar ways might be another way to help us do that. He sure succeeded in making us pay more attention to that little word act.