and little birds make melody that sleep all night with an open eye (so nature spurs them in their hearts):

Birds were said to sleep with an eye open in spring, too restless to close both. Skeat compares the modern “with one eye open.” Fowles is simply “birds,” as in the old phrase “fowls of the air”; the narrower sense, poultry, came later. Priketh is a rider’s word. Skeat’s Glossary gives the verb as “spurs” and “incites,” and later in the Prologue the Monk loves priking, hard riding (line 191). Nature spurs the birds in their corages, their hearts or inclinations; the word is our “courage,” which then meant the heart and what it wants. The same word comes back in line 22, where the narrator sets out “with ful devout corage.”