Q: So how DO the members of the Order of the Phoenix communicate
with each other?
A: I was surprised that this particular question won the poll,
because the answer (as I've already said) can be found in an
already-published book (Goblet of Fire), whereas the other two
questions related to book six. But perhaps I was influenced by the
fact that I knew the other two questions had interesting answers
and, of course, you will shortly know the answers to those
questions anyway!
Members of the Order use their Patronuses to communicate with each
other. They are the only wizards who know how to use their spirit
guardians in this way and they have been taught to do so by
Dumbledore (he invented this method of communication). The Patronus
is an immensely efficient messenger for several reasons: it is an
anti-Dark Arts device, which makes it highly resilient to
interference from Dark wizards; it is not hindered by physical
barriers; each Patronus is unique and distinctive, so that there is
never any doubt which Order member has sent it; nobody else can
conjure another person's Patronus, so there is no danger of false
messages being passed between Order members; nothing conspicuous
needs to be carried by the Order member to create a Patronus. -
jkrowling.com
And, as many of you have deduced, Dumbledore's Patronus is indeed a
phoenix.