First of all the blue one is Deimos and the red one is Phobos.
It so happens that 'Deimos' and 'Phobos' are the names of the moons that orbit Mars. The creator of Sailor Moon really looked into her astrology for this manga.
The two stayed in their crow form unless mars was in trouble. They first appeared in human form when the Dead Moon Circus captured Mars in Volume 12. The birds came to her rescuse and introduced themselves to her. They called her 'Princess Mars' and gave her the Mars Crystal. This helped her to transform into a stronger scout. yey.
They didn't appear again until Volume 16 when Sailor Lead Crow went to the Shrine. Then we find out a lot about the two 'birds'. They were from the plant Coronis, and so is Lead Crow. That is how Lead Crow knew where to find them (she used a crystal). Lead Crow tried to pass herself off as the true scout of Coronis b/c she had the Coronis Crystal, but the two knew that Sailor Coronis was the rightful scout. Lead Crow had gotten power from Galaxia, and could even use Sailor Coronis's attacks. She killed Deimos and Phobos right before Mars and Venus could come help.
~Back to The 'Birds' Shrine~