As everyone noticed, this game has way less combat than previous couple entries in the series and it can get kinda difficult to level your character since the only one viable way of doing it post game is hunting or starting new game+.
So if you're already done with a story and got bored of shooting those pesky birds to farm xp, or maybe you're still somewhere in a middle of it and need quick cash or couple skills unlocked for easier life, then you are in the right place.

Well, you go hunting. But instead of hunting birds/regular animals, you can force infinite spawns of special ones. As far as i noticed this way works only with rare birds.

• First you will require campfire "Kuwaq Yaku Ruins" unlocked. You reach this location early in a game, plus it's the first place where you get access to merchant. As you can see in a screenshot below that campfire is right next to one. And also it has a spawn point of eagle nearby.
  

• Another important thing is "Egle's Grasp" skill in you Seeker skill tree.
  In description in says that it increases the chance of finding rare animal, but it also doubles the amount of animals that spawn. In this case you can get 2 eagle spawns every time.

• Optionally you should equip any outfit/part that gives extra xp for hunting or, if you need extra loot, the one that increases the latter. Or simply 2 special outfit parts that grants both bonuses. If you don't have them yet, that's not much of a problem. Without boost you get 85 xp for one eagle kill and 95 xp for kill with boost. Item boost grants only one extra eagle feather pickup. I can't 100% confirm this, but i think Hoatzin's Hunger skill might also increase amount of feathers pickup, but i could not test it since i had that skill unlocked already. I'll update this if after new replay of a game.

What you wanna do is simply fast travel to this location. When you move couple meters towards temple entrance it will trigger eagles to spawn. As I mentioned above, if you got Eagle's Grasp unlocked, it's gonna be two birds all the time.
If they don't spawn initially you may restart checkpoint or travel to other location(s) for a while. But normally they are there pretty much always.
Now just kill them with a weapon of your choice. I recommend using pistol//rifle for faster process, but you can go with a bow if you like it. You will get 170-190xp depending on your oufit. Now if you want extra money you can run and loot eagles for feathers. Or if you need only xp then quickly turn around and use campfire, what forces game to save your progress. Now quickly back out of campfire and reload checkpoint via esc menu. Once again move couple meters towards temple and voila, eagles show up again. They'll keep respawning each time you reload cp. Simpy don't forget to save your progress by entering campfire each time as well. If you farm feathers too, once you fill your inventory, go to the merchant close by and sell them. It can net you a nice amount of 2,25k
From now on just rinse and repeat till you got enough xp/cash.

^