wenwell
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Indeed, the bird stash was disguised as a rock. I thought it was another clever mechanic, that if you don't have stashes, the portal will default to giving you birds.
If we don't have stashes, then creatures from the summoning portal can be obtained by capturing an enemy dungeon starting from the second week, and if that faction also has a captured stash. This applies not specifically to this map, but in general. If anyone has played the map Revelation, then you need to use this mechanic. It's a good map, but it's best to study the initial battles because artifacts and upgrades play a significant role.