In the spirit of zer83's Bloodletter spawn, here are mine about the Dominator.
The spawn pattern is slightly different for when the Dominator appeared for me.
The normal spawn will be 3 spiders, a set of 2 imps, 2 spiders then 2 birds.
When Dominator spawned the monster spawn order was 3 spiders, 2 spiders then 2 birds. You might even be able to see the Dominator as you are crossing the bridge, I think I saw it but I was in timeshift dodging the vulture at the bridge.
Hope this helps many of those looking for this elusive bird.