The most difficult alternative

The effectiveness of a level 4 unit depends on what you need, as monsters are purely defensive creatures, while thunderbirds are offensive.

Regarding resources: Very often, you have to build monsters because there aren't enough resources to build a stronghold (or you don't want to spend them :)), and waiting another 3-4 days isn't feasible. In general, all dwellings that are tied to the construction of a stronghold or castle are questionable. Very often, there are no funds to build them immediately, and there is no time to wait, so you have to build their alternatives.

Otherwise, birds are definitely more useful. Barbarians generally have an offensive tactic, and it is often much more important to engage enemy archers in melee combat early on than to expose a monster to their arrows to protect your own units.