An important rule adopted at the outset was that the words of an official birds name begin with capital letters. While this is contrary to the general rules of spelling for mammals, birds, insects, fish, and other life forms (i.e., use lowercase letters), the committee believed the initial capital to be preferable for the name of a bird species in an ornithological context, for two reasons.

  1. It has been the customary spelling in bird books for some years;
  2. Because it distinguishes a taxonomic species from a general description of a bird. Several species of sparrows could be described as "white-throated sparrows," but a "White-throated Sparrow" is a particular taxonomic species.

Jon S Greenlaw recently expanded this concept (in litt.) as follows:

The value of capitalizing the English names of animals seems obvious to me. 

Somewhere deep in the history of our language, we came to regard vernacular and "common" names as second-class citizens. Thou shalt not capitalize them.

I believe that it has something to do with the (mis-)perception of species by folks in the humanities as "categories" or "classes" rather than as real entities (to the extent that we can know them). Well, biology has come a long way from that typological view. The standardized English names now have graduated from the realm of "common/vernacular" names.

From my perspective, the strongest argument for capitalizing the English names of birds is that we now have a single, unique name (see below) for each of the biological entities that we call bird species. These names must be regarded as proper nouns (thus receive capitals in all English publications), rather than as common nouns (vernacular names). My unabridged dictionary defines a "proper noun" as (1) a word that is not necessarily preceded by an article (e.g., "the," "a") and (2) denotes a particular person, place, or thing.

A species is a particular thing or biological entity. We have other proper nouns in the English language that are composites as well. We talk about the "Great Lakes," the "Rocky Mountains," and the "Alps."   In the same way, "American Robin" says it all.  No "the" is necessary except as grammatical stricture in a sentence may require one for proper nouns.