The Chicago Manual of Style has several entries on this (7.107 - 110, Names and Terms / Scientific Terminology / Vernacular Names of Plants and Animals).  To excerpt:

"Common names of plants and animals are capitalized in a bewildering variety of ways, even in lists and catalogs having professional status.... In the absence of [an 'official'] list the University of Chicago Press recommends a <i>down</i> style for names of wild plants and animals, capitalizing only proper nouns and adjectives used with their original reference, or suggests consulting a standard dictionary."  (emphasis in the original)

They give a number of examples, including "Cooper's hawk" and "Canadian thistle" -- "Cooper" being capitalized as a proper name, "Canadian" as a geographic entity -- as well as rhesus monkey and black bass -- no capitalization necessary.  The Manual allows exceptions for domestic breeds that have been given official names.

Thus "house sparrow" or "English sparrow," rather than "House Sparrow" or "English Sparrow," spellings confirmed by both Random House and Webster's.

-- Eugene Dillenburg
Exhibit Developer
Science Museum of Minnesota

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