An Additional Note on Walt Whitman

John Burroughs, in his 1867 Notes on Walt Whitman as Poet and Person (section XXXI) employs three birds in his account of the necessity of encountering objects in the fullness and richness of their surroundings and histories:

It must be ever present to the true artist in his attempt to report Nature, that every object as it stands in the sequence of cause and effect has a history which involves its surroundings, and that the depth of the interest which it awakens in us is in proportion as its integrity in this respect is preserved. In Nature we are prepared for any opulence of color, or vegetation, or freak of form, or display of any kind, by the preponderance of the common, ever-present features of the earth. I never knew how beautiful a red-bird was till I saw one darting through the recesses of a shaggy old hemlock wood. In like manner the bird of the naturalist can never interest us like the thrush the farm boy heard singing in the cedars at twilight as he drove the cows to pasture, or like the swallow that flew gleefully in the air above him as he picked the stones from the early May meadow.

Why might Burroughs, close friend of Whitman, have insisted on a sequence of three birds to illustrate his point? Theories and insights are welcome.