What I would do is this:
- Do the players specifically state that they are looking up there, shining their light up there?
- If so, I would probably give them a 3-in-6 chance to spot the apes, modified (plus/minus) by 1, depending on how they described their vigilance, and perhaps more if they have something crazy like a spyglass.
- If the players have failed to notice the apes (bad roll) or they have not stated that their characters are specifically directing their light and their vigilance in the specific direction of the winged carnivorous apes, I would go straight to a surprize check. Due to the apes being above head, and the beneficiaries of being cloaked by darkness, I would improve the surprize chance from 2-in-6 to 4-in-6. Alertness ability (rangers, et al.) can also modify that roll.
HYPERBOREA- A Role-Playing Game of Swords, Sorcery, and Weird Science-Fantasy