Possibly my most played game at over 600 hours, (25 days)
I really really love RDR2

Not only is it the best cowboy game of all time,
with a story-line that famously makes grown men cry,
it also manages to be one of the best
horse games, hunting games, and fishing games of all time too.

It's also probably the best wildlife watching simulator,
with insane attention to detail on animal behaviours and interactions.

While just roaming the wilds, I have seen an eagle dive out of the sky, take a fish from a lake
and fly away with the fish in its talons

I have watched two stags get into a fight, then subsequently get their antlers locked together and panic.

Other things nature nerds might appreciate:

You can watch a cougar stalk a deer through the grass
You can get charged by a black bear, call it's bluff and hold your ground,
and the bear will walk away. And true to life, this won't work on brown bears.
Wolves can be seen playing with each other,
and if one of their pack is killed,
they can be seen howling their grief
Possums will play dead when you approach
There is a bird called the Carolina Parakeet in the game,
it is very difficult to find because its so rare.
They don't respawn, and if you kill 25 of them the species goes extinct.
The Carolina Parakeet really did go extinct in the 1920's
Similarly, time progresses as you move through the story,
and as the frontier becomes more and more settled, the wildlife noticeably diminishes

Sketches

As you go around discovering new places and animals,
Arthur sketches them down in his Journal, which is a thing of beauty by the end of the game.

This, like i'm sure it has done for many others,
is what inspired me to try my hand at nature journalling

I will upload some of my sketches here