Here’s a few options on how to flesh out the 1d4 Rocs from my Random Mountain Encounters.
1) Recent Kill
The characters are passing along a trail below a cliff, when a whole elk drops onto the trail in front of them, glancing up, they see a huge Roc dropping from the cliff overhead diving towards them to retrieve the kill that it dropped. They have 1 round before the beast will be upon them,
2) A hunting pair
The characters are crossing an open stretch of rocks between one forested section of mountain trail and the next. A sharp-eyed PC spots what looks to be one very large bird nearby, and approaching. By the time they realize that it’s actually a really large bird that was far off, and closing on their position, they are too far from either wooded section of trail to find cover there.
What the PCs don’t realize is that atop a rocky outcropping just north of their position is the nest that two Rocs share, and the male sitting on a pair of eggs which have been laid only days ago. The Rocs have not yet spotted the PCs, but they will do so quickly unless they can take some action. This pair will defend their eggs, but are not currently hungry, having just returned from a hunt.
3) Learning to Fly
The party’s morning routine is disrupted when several of the large trees at the edge of camp crack and splinter, as a juvenile Roc lands roughly on them having just left the nest for the first time. Her proud parents are following just behind, and land on two other sides of camp. All three enormous birds eye the characters, deciding whether flying is hungry enough work to justify the work of devouring the PCs, or maybe just their horses.
4) The Abandoned Eyrie
Centuries ago, a group of frost giants kept an eyrie and raised Rocs for use in battles against the Dragons. However, shifts in the ordning eventually disgraced the family and forced them to move on from their mountain home. They left the huge stone eyrie that they built to house their “pets” and simply released the beasts into the wild.
In the centuries since, two families of Rocs have continued to occupy the eyrie, while most of the buildings and structures on the mountaintop decayed into ruin. Now, the only remaining structure is a stone building, with one side open to the elements where ancient ironwood timbers serves as roosts for the two mated pairs of Rocs who reside there. Each pair has their own separate “nesting box” from which they cannot see eachother, which is fortunate otherwise they would almost certainly kill eachother.
Exploring the ruins of the erie might reveal some treasures left behind by the giants when they left the place, but who knows what else has taken up residence in the giants' decaying root cellar.