I only recently installed Pets and almost immediately came upon a yellow naped amazon which my sim befriended, and later on a falcon. But other than those two I haven't been able to even TRY to catch any large birds. I don't even get an option to befriend them, only "watch". I've looked into the hidden skill related to befriending animals, I even used a mod to max it out, thinking that would fix the issue but nothing helped. So why is it that I could catch two rare (or uncommon?) birds right away but now I can only watch them, even if they're more common, or even if they're the same exact birds as I've caught before?

I've read that having seeds in your inventory might cause issues, so I removed all seeds but it didn't work. I've also read that this problem only occurs on community lots but the amazon I first caught had spawned in a park lot, so that might not be right either.

I used to get this now and then as well. I assumed it was a mod conflict. My game would start out normal and then gradually, birds would start to spawn un-catch-able. Then I couldn't catch any birds at all. I recently cleaned out my mods and only put back in the few I really needed. The problem seems to have disappeared.

It's a fact. With a bird that is on someone else's residential or community lot, unfortunately, you can not be friends - this option is always not available. And if you find a bird in your own yard or on "neutral" territory outside the lots, for example, on a hill, you can try to take it. Maybe the amazon you first caught had spawned near the park lot?

Looks like EA flipped us the bird on this one then.

According to the BeFriend interaction script, you can only BeFriend when the bird isn't captured.

Yeah, I just read that in the code too. It seems that it's the only restriction why we can't befriend a bird. But what the heck makes those birds "already captured"? They're basically straying in the wild... it shouldn't have been set "captured"!

After some additional study, I notice that this scenario can happen when the MinorPet Escape from the cage (he is still 'captured', however "straying in the wild").

Conclusion: maybe the OP is trying to BeFriend an escaped bird.

Sometimes, when I try to capture a wild bird, the option won't come up. Once the spawner changes, sometimes I'm able to adopt one.
I've been able to adopt birds in the world, on community lots, on my own lots...
Still unsure why EA would have a wild bird be unable to be 'befriended.'

So does that mean that NPC sims catch minor pets too and sometimes they escape?
Is there maybe a mod or something to make any animal adoptable?

No exactly. Human NPCs aren't allowed to catch them autonomously, but Cat does because it's attached to the hunting skill.

Are you sure about that? I looked at Befriend_Bird.ITUN, and it seems that no Sims are restricted from befriending wild birds. Nothing advertises it anyway, so it might not be very common. But I'm very sure I've seen some of the residents or townies play with their own parrots and birds, so they could've possibly run away and those uncapturable birds originally belonged to a townie, and that actually makes sense. Maybe StoryProgression pushed them to collect the wild birds in the first place, though. I'm not sure about that.

I am not too familiar with the autonomy system, but if the interaction don't have advertises autonomous sims will avoid it. However, Cats can hunt and capture birds autonomously this will change the "Capture" flag, avoiding people to befriend them.