My opponent has no poison counters. If I attack with any infect creature and {thrumming bird}, assuming my infect creature deals the damage to my opponent, does my proliferate work? Both creatures deal combat damage at the same time so I'm not sure if I can proliferate that poison counter same turn. Help?
Yes. Both creatures deal damage. Thrummingbird deals regular combat damage and it's ability goes on the stack. The other creature deals damage and opponent gets poison counters. If nothing negates the proliferate ability of thrummingbird, then that resolves and you can choose to have him get another poison counter
Awesome. Thanks!