[RPG] How to determine passive insight/perception

In and of itself, a bird on a roof is not something out of the ordinary, and not something that would (or should) alarm a character.

I assume from the fact that you are asking the question, that by noticing the bird, and divining its purpose, the character can gain some advantage in the near future.

So to me, this isn't even a perception question, but an insight one - I would let the bird be seen, almost automatically, but only address its significance if the PC passes a passive (or active) insight check.

I would ask the player if they are keeping an eye out for trouble or anything else in particular. If they claim to be, ask for details of what they are looking for, and then give answers appropriate to the questions that are asked.

DM: You proceed down the path towards the building. The light of your lantern illuminates the way ahead, but darkness pervades on either side of you. Do you make right for the building, or do you want to play it safe a study your surroundings?

PC: Play it safe of course. What do I see?

DM: Well, there are some trees on your right. You hear the rustle of what might be wings of a small bird pass overhead. Are you looking for anything in particular?

PC: Well, am I sure the noise is just a bird and not something bigger? Do I see or hear anything threatening?

DM: You're not totally sure about the sound, but you don't see or hear anything threatening.

...

If your players are going to feel betrayed when the significance of the bird is sprung on them, have them hear a bird call as well as the bird above, and then if they ask about birds, let them see the one on the roof, and maybe another in a tree.

If your players don't go for the idea of player skill (embodied here by the need for the players to divine that birds are significant), then a similar scene can be played out, bringing passive insight (or perception) into play by giving out details if their passive score beats a medium or high difficulty for their level.

Picture a suspenseful scene in a film - as the viewer you would expect to be aware of the birds before they are meaningful, but you would either not expect to know the role they have to play, or not expect to know which bird it is you need to focus on unless you have already learned the significance of birds.

As for what to compare the PC's active or passive perception or insight against, the Rules Compendium p126 has the difficulty class by level chart.

I would suggest that this is medium or hard to detect (as significant) for a low level character - so I would pick hard for level 5, which is a target number of 22.

Well. There is much advice and hints and tips and tricks to be learned. But I will keep on task, and I will help you in your specific situation. You can ask more questions later, relating to any other aspect of running a game that you want.

The Journey

The start of the ambush begins as the players first start on their way. You describe their surroundings, what kind of path, or road they are traveling, you ask them to describe how they are traveling, what things they bring with them, etc... because obviously as the GM you should know what your players are up to, how they are dressed, how they prepare themselves, etc. This lets the players describe to you their preparedness without them knowing they are doing that in preparation for an ambush.

Example: The players are just packing up their camp on their second morning on the road between two towns. You describe the path to them as muddy from the nights rains, and the scattered trees as glistening with rain drops, the breeze is sharp and slightly cold, and the sun is still rising over the horizon, You ask them to describe how they dress, how they prepare, because after all - the journey awaits. The warrior describes to you how he is wearing his armor and cape, but that he puts his bow in the wagon because he fears it getting dirty on the mud if he walks with it, and the wizard explains how since his robes are now soaking wet, he switches to his traveling gear (which incidentally, don't have his secret pocket full of alchemist fire flasks). etc etc.....

Misdirection

Here is an important part of the ambush, you want to check if maybe your players characters can detect the ambush, but you must do it in a way that they do not feel they are about to walk into an ambush. You want them to roll their perception (spot, listen, smelling, 6th sense - whatever) without knowing its for an ambush, and also after the roll, they should be feeling safe and unsuspecting.

Example: You describe to the players how they spot the carcass of a dead deer on the path ahead, and ask them to roll their perception. You generously describe the deer and its surroundings to those with higher rolls, and mention that it poses no threat since they can clearly see the animal was killed by a hunter who took its antlers. The players who may have initially been concerned with a dead body, are now calm and unafraid. And you have their perception rolls for the scene.

The Twitch

Something is not quite right. Some of the players may start to be suspicious, this indicates that the players have caught on to the fact that something may be afoot. Preferably this happens after you have already got everything in place, if not - you need to act fast before the ambush is too expected. Regardless, this is the point that you will be at right before The Reveal.

Example: A player may ask you "Will anything interesting happen here? Why are you describing to us this boring forest as we walk by?" Or "Dude, this is totally an ambush!" (okay, I don't have amazing examples)

The Reveal

This is the moment you have been building towards, it needs to be grand and really make your players feel surprised. Regardless, once the ambush is revealed, your deception is pretty much over, and combat (or fleeing) will begin.

Example: Maybe an arrow suddenly flies past them, narrowly missing them, signifying the start of the ambush. Or perhaps the shouts and screams of a dozen Kobolds suddenly surround them from all sides as they charge forth from the forests concealment.

On The Table

About what you do physically: Only pull out the map after the ambush has been revealed. Until then this is just a normal day traveling the road, and you describing the wonderful scenery. Based on their perception rolls and how they describe their characters, you will judge their preparedness (including their initiative rolls) to see if they can draw weapons, or prepare anything else for the ambush.