Clockwork Avian enters the battlefield with four +1/+0 counters on it
Clockwork Avian can't have more than four +1/+0 counters on it.
At end of combat, if Clockwork Avian attacked or blocked this combat, remove a +1/+0 counter from it.
X, T: Put up to X +1/+0 counters on Clockwork Avian. This ability can't cause the total number of +1/+0 counters on Clockwork Avian to be greater than four. Activate this ability only during your upkeep.