Passive mobs (cows, pigs, sheep, chickens) sometimes do not spawn

I made a world on snapshot 21w39a and I've also had the same issue except I've explored a few thousand blocks and haven't seen a single passive mob

I mapped the passive mobs with the 202109291427 seed.  You can see that there are regions where the density looks almost normal, and other regions, large regions, where the density is zero.  The background is black where there is water to indicate that the zero density regions are not over oceans.  Maybe Mojang is hinting at a vegetarian future 🙂.  Depending on where you spawn you could have the impression of a very empty world.

Issue is still present in 21w40a.  The attached map of passive mobs is for seed = 3. Land is brown, water is dark blue. Note that there are plenty of mobs in the upper right and almost none in the lower left.

Using 21w41a I generated 13  worlds with seeds 100 to 112 inclusive and analyzed the passive mob locations.  If account is taken of ocean, desert, badlands, dark_forest, and old_growth_pine_taiga, and snowy_plains, the distribution appears to be fairly uniform.  Possibly the new random number generator in 21w41a fixed the problem.  It would be interesting to hear if anyone else still sees large areas without passive mobs when using 21w41a.

I can confirm that on my world as of today on snapshot 20w42a, cows, pigs, sheep and chickens do not spawn in jungles or dark oak forests. I am however seeing a lot of ocelots, so it does not affect all passive mobs.

I can't confirm. I could find parrots, sheep and cows in my world generated at 20w41a. Jungles and Dark Oak Forests usually have less mobs due to the reduced area mobs are able to spawn.

Note that the list of what animals spawn in what biomes has been tweaked slightly. Make sure you're not looking for cows in biomes they don't spawn in at all for example. But this shouldn't affect any tests that just look for passive animals regardless of species.

And animals are less likely to spawn in:

• Dark Forests because the animals require a light level of 8 or higher to spawn.

• Jungles because they also require grass_block as the surface they spawn on, which Jungles block with lots of underbrush. Animals used to get around that by spawning INSIDE leaves, but that was declared a bug and eventually got fixed. Not sure what the Ocelots are doing to get around the problem.

Seed = 100,  version 21w43a, survival mode.

Cows, pigs, sheep, and chickens require grass blocks and, as you point out, a sufficient light level to spawn.

The first image is a crude map of biomes.  Light green represents some form of grasslands, dark green represents some form of forest forest.  The white is some kind of snowy biome.   The black dots are either cows, pigs, sheep, or chickens.  Notice that from X = 0 to X = 1024 and Y = 3100 and Y = 3700 the number of entities drops nearly to zero.   Elsewhere the entities generate normally in both grassland and forest.

The second image is a map of grass blocks with cows, pigs, sheep, and chickens marked in red.  You might need to blow up the images to see clearly.  Grass blocks under trees were removed to make sure light level was not an issue.  There is a huge area of grass blocks without any entities from X = 0 to 1024 and Y = 3100 to 3700.

Starting with 21w41a this issue occurs much less frequently, but it still occurs.  I had to search for it in this world.  I would be very curious to know how the algorithm works because it is not so easy to accidentally write an algorithm that excludes large areas but works perfectly in others.