Well i think in these cases character knowledge is important. If they know about sturges,then yes the idea makes sense. If they see stirges feed they may be able to out two and two together, so the idea makes some sense. If they have encountered them before, it also makes sense. Same with trolls.
This is where I, as a the DM, take responsibility for honestly portraying the world the PCs are interacting with and exploring.
I don't say, "There are stirges! Roll for initiative!" That's ridiculous.
I want to reflect a monster's ecology, abilities and potential weaknesses in their behavior and habitat.
It's the same thing I do with traps. A crushing wall trap? I place descriptors of the grated floor or grooves. A stirge? I place animal remains with obvious drain marks and so on.
The PCs look around and examine things, they are rewarded for interacting with the environment by learning about the world through these sort of descriptions. It's not meta-gaming, because the first-time player has just as much chance of looking around as the old timer. The old timer just knows to ask already, and so the new guy learns to do the same. It's a nice feedback loop.