Dumbledore's Silvery Sign

[annemehr]

Entropy brought up this passage:
> >> "Should I go get someone?" said Harry. "Madam Pomfrey?"
> "No," said Dumbledore swiftly. "Stay here."
> He raised his want into the air and pointed it in the direction of
> Hagrid's cabin.  Harry saw something silvery dart out of it and streak
> away through the trees like a ghostly bird.  (GoF, ch 28)<<
[...]

Well, as it was sent towards Hagrid's cabin, I never gave it a second
thought but assumed that's what summoned him.  Hagrid apparently knew
it was urgent, too, since he appeared running, out of breath, and with
Fang and the infamous crossbow.

I hope I'm not "cheating" posting this, but I believe JKR has told us
the silvery thing was indeed a bird, and the kind you would expect. 
>From the Edinburgh Book Festival:
---------------------------------------------------------
Q: What form does Dumbledore's Patronus take?

JKR: Good question. Can anyone guess? You have had a clue.  There was
a little whisper there.  It is a phoenix, which is very representative
of Dumbledore for reasons that I am sure you can guess.
----------------------------------------------------------
I'm sure the "whisper" she refers to is the passage Entropy is posting
about.

What I'm not sure of though is whether you all would agree.  After
all, we'd never had any other indication that a wizard could send a
Patronus off to run errands! How would it have communicated to Hagrid?
 Would it have just been a prearranged understanding between
Dumbledore and Hagrid that Hagrid would always go to him if ever the
Patronus was sent?  Why then, in OoP, does Dumbledore always use
Fawkes himself to send messages?  Could Harry learn to do something
like this, or would this be one of the things Dumbledore does with his
wand that Madam Marchbanks had never seen before? (And I *heard* that!
Quit sniggering!)

Annemehr