The speaker starts off with a wish: he wants to be back in England, now that it's April. Anyone who's there now would be waking up to see beautiful elm trees and song birds, like the "chaffinch."
The speaker also tells us that next month, in May, an England-dweller would be able to check out birds like whitethroats and swallows and thrushes. He or she would also enjoy the blossomed branches of the pear trees and the fields full of buttercups.
In the meantime, the speaker is stuck here, just contemplating a "gaudy melon-flower"—and he doesn't sound thrilled about it.