Like the title implies for the basis of this suggestion, a proposed tag :
[ALTITUDE:Height Value X] determines the recommended height at which a creature is egible to spawn in and preferably leave, leaving is less important as long as its the medium (water, land access, air, cavern entrance) that they generally entered and are comfortable with though the will go through the most preferable exit.
For birds like fliers, this would determine the
recommended set z level height at which they spawn, with some birds being closer to the treeline and others being high soarers like albatrosses & eagles. This should hopefully streamline spawning and hunting attempts by dwarves, and help create predictable patterns that may make 'flight path traps' a little easier. A blob of ravens blocking spawns at 30 z in the air randomly rising and lowering in the map is not good when instead they can stay comfortable at a idling altitude then make a plan to leave at a exitpoint clearly.
- Pest & vermin birds will often fly closer to the ground, making them easier to pick off with a crossbow, and birds may have a altitude low enough to leave directly at ground level and naturally stay and linger on the ground/trees relative to surroundings, say if a eagle perches on a high mountain for instance in its z level range.
- Fish that spawn in water can also be given negative altitudes for deep sea monsters like giant squid, like birds this streamlines their exit points & should reign in behaviour of bobbing around by making them prefer to mingle naturally at a certain depth so they can move directionally on a X plane rather than Y through multiple z levels.
Anything that does not have a [ALTITUDE] tag is counted as exempt from it, and additional exceptions like [ALTITUDE_EXEMPTION:BIOME] for yeti's to not require to be a minimum height of 20-50 z levels high as a biome in a glacier compared to a mountain for a practical example. Creatures who can't find map conditions fitting the [ALTITUDE] they need will be much sparser in their respective biome.
This system is not meant to replace subterrenean biome listings but may prove interesting for putting power in the player's hands to change the egible spawning positions.