How do they manage an average of 679 damage?
First Aerial bombardment rules would probably give the Tarrasque a DC 15 Reflex save for half damage for each. Assuming it was a surprise at first the Tarrasque probably doesn't get this so I'll ignore it.
Second, a Giant owl's likely only weigh like 140lbs by loose calculation, being a little over 4x the height of a snowy owl (so assuming 4 times equivalent weight and then cubed is 64kg which approximately equals 141lbs. It could be a little higher but its not breaking 200lbs) and requiring falling at least 20ft before they even start ranking damage by the srd 3.5 rules for items falling on players (https://www.d20srd.org/srd/environment.htm). Assuming you meant 40ft over the Tarrasque, and allowing for 1d6 damage every 10ft past the point instead of the 20ft that's implied to be required, the owls would deal 2d6 damage each at that height, requiring 20ft of falling to start incurring damage. Even without it that's not 679 damage.
That's pretty much 0 damage too, because 2d6 per owl - subtract the DR 15 of the tarrasque from each instance of damage - is 0 damage. Iirc there was a min 1 damage even for negative strength modifiers but DR superseded that. Even if I'm wrong that's 1 damage per owl max.
Even if you went the 220ft up above the Tarrasque you'd need to hit maximum fall speed under the more polite 1d6/10ft rules, after falling 20ft, you'd end up with 20d6 each, the cap for fall damage. Which after DR is 440 damage.560 damage without DR.
Which actually isn't that high up. I thought the Tarrasque was taller than 50ft, but its still a hell of a timed shot tbh. It assumes the Tarrasque doesn't move for like 6 or 7 rounds, or moves in a straight line into the falling birds.
That doesn't' fix the weakness of a Tarrasque to some form of high impact drop damage, necessarily, just means that I'm suspicious the birds can pull it off.
That's fair. Neither is the Tarrasque.