Big Questions, Big Nature, Big Beats
A good deal of Auden's poetry deals with Big Issues: political, social, and psychological. Nature imagery shows up quite a bit, too, and he tends to go big. You'll certainly find a fair share of birds and beasts, but Auden isn't afraid to throw in large scope nature as well: seas, seasons, mountains, glaciers, deserts, floods and the like.
Auden was a master of form, so that means lots of rhyme schemes and regular meter.
Take a look at "The Wanderer," "Lullaby," "In Memory of W.B. Yeats," and "In Memory of Sigmond Freud" to see more of W.H. being W.H.