“What’s that, Strider? It don’t look like a cloud,” said Sam in a whisper to Aragorn. He made no answer, he was gazing intently at the sky; but before long Sam could see for himself what was approaching. Flocks of birds, flying at great speed, were wheeling and circling, and traversing all the land as if they were searching for something; and they were steadily drawing nearer…
From THE FELLOWSHIP OF THE RING
by J.R.R. Tolkien
No. Enc.: 0 (1)
Movement: 10 (3)
Fly: 150 (50)
Armor Class: 5 
Hit Dice: N/A
Attacks: 1 per target within area of effect (beaks and claws)
Save: F8 (for the flock)
Hoard Class: None
This represents a huge flock of aggressive birds intent on attacking. Every target within the flock’s area of effect (see below) is subject to one attack per round.
The relentless, amorphous nature of the birds can overwhelm those who fall victim to it. Targets within the flock must make ineffective attacks against the birds, allowing only combat movement rate, unless they make a successful save vs. paralyzation. A roll of ‘1’ on this save means the character panics and stops in place and must successfully save to get moving again. A character’s saving throw adjustment for wisdom applies to these rolls.
Area of effect for each size of flock:
Small: 100 + 1d100 (101-200) birds in a flock roughly 50′ across
Medium: 250 + 2d100 (252-450) birds in a flock roughly 100′ across
Large: 500 + 5d100 (505-1000) birds in a flock roughly 300′ across
Huge: 750 + 1d100x10 birds (760-1750) in a flock roughly 500′ across
Gargantuan: 2500 + 1d20x100 (2600-4500) or more birds in a flock roughly 1000′ across or more
Hitchcockian: 5000 + 5d100x100 (5500-55000) birds in a flock a mile across or more
Attacks against the flock will kill some individual birds but have no measurable effect on the flock. Area effect (fireball, cone of cold, breath weapon, etc.) attacks are an exception. The flock is constantly moving with a group mentality, which will make effective combat against it very difficult.
You do not want to run into The Birds.
I just pulled the numbers for flock size out of the air. If anyone has suggestions for improvement, chime in.
UPDATE: Get your own flock: Muster The Birds.