One thing that surprised me when reading up on eagles in Wikipedia for Roc comparisons is that, while they do tend to go for prey they can carry away, some eagles actually take on prey several times larger than themselves (such as small deer), killing them in situ and then removing pieces to carry away.
With that in mind, Rocs might well occasionally go after prey even larger than themselves, especially if hunting in pairs.
Well the
Monstrous Manual says Rocs eat Purple Worms, who are larger and significantly heavier than a Roc. For that matter, an Elephant may be a size category smaller but it'll often be heavier than the 8000 pound average bodyweight a Roc has in the 3E SRD.
It'd make sense if they dealt with such dangerous prey by attacking them in pairs.
Like real-world wild predators, Rocs have to balance the effort and risk of attacking an animal with the nutritional value of the prey. Rocs don't have
wands of cure light wounds like adventurers, or even allies with the Heal skill. If they get wounded, they have to rely on natural healing - which'll be 18 hit points per day for a D&D 3E Roc, or 16 for a Pathfinder Roc. If a Roc is taking more damage than that on average to kill its prey, it's likely to be slowly nickle-and-dimed to death.
Elephants in D&D are
nasty. Let's compare their stats:
SRD 3.5
Roc: AC 17 (-4 size, +2 Dex, +9 natural), touch 8, flat-footed 15; hp 207 (18d8+126), Melee 2 talons +21 melee (2d6+12) and bite +19 melee (2d8+6); Power Attack; Hide -3, Listen +10, Spot +14;
Carrying Capacity (flying) 7456 lbs.
Elephant: AC 15 (-2 size, +7 natural), touch 8, flat-footed 15; hp 104 (11d8+55), Melee Slam +16 melee (2d6+10) and 2 stamps +11 melee (2d6+5); or gore +16 melee (2d8+15); Listen +12, Spot +10
Pathfinder
Roc: AC 22, touch 8, flat-footed 20 (+2 Dex, +14 natural, –4 size); hp 120 (16d8+48), Melee 2 talons +18 (2d6+9/19–20 plus grab), bite +17 (2d8+9); Power Attack [-4 melee, +8 damage]; Fly +7, Perception +15;
Carrying Capacity (flying) 3200 lbs.
Elephant: AC 17, touch 8, flat-footed 17 (+9 natural, –2 size); hp 93 (11d8+44), Melee gore +16 (2d8+10), slam +16 (2d6+10); Power Attack [-3 melee, +6 damage]; Perception +21
Going by those numbers, an Elephant will generally notice the Roc coming (Spot +10 or Perception +21 versus Hide -3 isn't much of a contest), and a single Roc will need a couple of rounds to kill an Elephant even if it uses Power Attack unless it makes
exceptionally good rolls.
In two rounds, an Elephant can do a lot of damage - the D&D version does 34 points on average if it hits with its slam and both stomps, and the Pathfinder version averages 36 with a slam and gore. Worse, the Pathfinder elephant has Power Attack, which could increase its damage output significantly. And Pathfinder Rocs only have 120 hit points, so it doesn't need to be
that unlucky to lose a fight or three and get killed if it lives by hunting elephants. Furthermore, Elephants live in herds and are none to actively defend their kin - an unlucky or inexperienced Roc that allows itself to get attacked (or, possibly worse,
grabbed) by three or four Elephants is likely to get itself killed.
A pair of Rocs would fare much better. Two 3E Rocs using Power Attacks in a dive average enough damage with their talon attacks to kill an elephant, and they're not likely to miss. If they win initiative they can kill their prey before they risk injury themselves.
Furthermore, a pair of Rocs can carry most of the meat home - a Pathfinder Roc can fly with a 3200 pound load, so they can only carry enough prey to feed a nestful of young for two days.