Clockwork Avian [Antiquities]

Flying Clockwork Avian enters the battlefield with 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.