In superflats where the biome is set to jungle, the game will spawn hundreds of parrots a minute, without limiting itself past a certain entity count. This will result in thousands of parrots in the world within minutes.

With extra testing, here is my theory for what is happening. Parrots, like ocelots, use the hostile spawning algorithm to spawn in jungles. However, neither of them actually contribute to the hostile mob cap, meaning their spawning is theoretically unlimited. This is however, a much smaller and less noticeable problem with ocelots because of these differences:

• Ocelots eventually despawn; parrots never will.
• Ocelots seem to spawn in smaller "packs" than parrots, a pack of parrots seems to create around 6 or 7 parrots each
• According to the wiki, spawning an ocelot has a 1/3 chance of failure; I have no way of measuring the failure rate of parrot spawns, but it does not seem as restrained as ocelot spawns.
  Despite this, it seems that ocelots still spawn faster than they naturally despawn, and since they are not capped, they too can possibly overrun the world, just at a fraction of the rate parrots do.

Notice that the game will stop spawning ocelots and parrots when the game is set to peaceful or when the hostile mob cap is filled (tested with ~80 shulkers). As soon as parrot spawning stops, animals will be able to spawn. (Suggests that parrots actually occupy the passive mob cap)

Steps to reproduce:
1. Create a jungle superflat (Sample preset: 3;minecraft:bedrock,2*minecraft:dirt,minecraft:grass;21;village)
2. Spend a few minutes in the world, you will notice fps beginning to drop as more and more parrots spawn.
3. Fly around and notice parrots dominate the area; they should be in their thousands within five minutes or less

Possible ways to fix it:

• Make untamed parrots despawn like ocelots
• Change the spawning algorithm to check the existing number of parrots before spawning more
• Make the game spawn fewer parrots
• Have parrots spawn similarly to other animals, with the ability to breed them.