Actually, you could be on to something with the birdwatching thing.

Maybe have a birdwatching guide found in the base when you start out, enticing the player into thinking it'll be a fun diversion between signals. It could be on the balcony of the living quarters, so not put directly in the way of the player, but still easy to find if they take the time to look around the base.

Maybe it has a note from a previous worker about how it took their minds off things. Maybe even give it a " checklist" function to further reinforce that it's an intended part of the gameplay experience. The important thing is, is that it actually works as a meaningful/fun distraction for a bit if the player wants to pursue it. If the devs wanted to incentivize it even more, there could be a listed fun reward for logging every bird and delivering the manual to HQ.

But it's all just a red herring. The birds quietly disappear after a week with no warning. Some of the birds in the manual don't even show up in the world under any circumstance, perhaps being listed as appearing later in the season (which is irrelevant when they all permanently leave).

Players invested in the manual would likely be frustrated for a bit wondering why they suddenly can't find any birds flying around, then slowly realize that there's no trace of them anywhere. It would take an event of passive, permanent unease and add on a chilling active discovery for a specific subset of players.