Correct me if I'm wrong, but in this event, we have the same drop rate on all levels (on phase 1) for the summoning material
Event 1-1
Stamina 10
1 feathers 0.1f/S
Event 1-2
Stamina 20
2 feathers 0.1f/S
Event 1-3
Stamina 40
4 feathers 0.1f/S
Event 1-4
Stamina 80
8 feathers 0.1f/S
Also, no bonus levels