Why are common names capitalized here?

Despite your search,

here are some supporting examples.
Odonata Central, common names are capitalized.
Bamona, common names are capitalized.
Ohio Spiders, common names are capitalized.
Apparently it is a rule made to be broken.

… john and jane balaban, 16 November, 2012 - 9:36pm

Inconsistent

A name is a proper noun and wants capitals. A descriptive term is not. "Greater Milkweed Annihilator" could be treated as a name or a descriptive term.

I generally capitalize page titles. I often use normal writing for descriptive text. Call me inconsistent if you like. I don't know of a style guide.

Common names of birds, butterflies, and dragonflies have more formal, proper name-like status due to checklists and people who will scold you for not calling that white-bellied woodpecker a "Red-Bellied Woodpecker." I consider these not truly "common" names but specific names written in English instead of Latin. No real person talks about variegated meadowhawks.

When we get into flies we can do anything we like. House Fly, house fly, housefly, house-fly. (The two word rule on the Flies page is not a real rule.) Arguably "house fly" does not even name a species, but is a descriptive phrase for a variety of large, brown flies which may be found indoors.

… John F. Carr, 16 November, 2012 - 8:22pm

exactly

one can find any number of sources where the English names of species are consistently capitalized for the avoidance of doubt (i.e., 'House Fly' to refer to Musca domestica, to the exclusion of other spp.); lowercase phrases like 'two-spotted ladybug' always leave too much room to [mis]interpretation.
on BG, it was agreed at some point to keep the 'common names' capitalized on the info pages; on the other hand, in comments more casual practices prevail.
all in all, this lack of consistency is another reason why scientific names should be given preference in any contexts where organism identity is important.
again, as John has remarked, in some groups the English names are effectively scientific -- but there are pitfalls in that area, too: e.g., in Odonata, the same species may have different 'standardized' English names in the UK and the US, and even in the US there used to be more than one full set of English names, as far as i know (for Plecoptera, too).
unlike true scientific names regulated worldwide in a centralized manner, the authority over common names is basically up for grabs -- because taxonomists don't really care about such names (nor use them in technical papers), so anybody can try to coin a catchy name for this or that species in their native tongue, or even undertake to assign such names to every member of a major taxon. a county horticulturist association may come up with a list of names for every mite species there is; those who fancy those names may adopt them, others are free to disregard... none of this would be binding on anybody, or of any consequence to the scientific nomenclature.

"No real person talks about variegated meadowhawks." --John F. Carr
ueberquotable.

… v belov, 17 November, 2012 - 10:59am

Common names

Name or descriptive term, these are common names. I don't have the time to find a better resource, but a 1-minute search produced this guide and this page that dictates that common names are lower case unless a word is a proper noun.

… RupeeRoundhouse, 16 November, 2012 - 8:57pm