Clockwork Avian

Flying
Clockwork Avian enters 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 only during your upkeep.

Masters Edition IV

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.

Beatdown Box Set

Clockwork Avian comes into play 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 turn, remove a +1/+0 counter from it.
o
X, oc
T: Put X +1/+0 counters on Clockwork Avian. Play this ability only during your upkeep.

Rinascimento

Renaissance

Fourth Edition

Flying
When Clockwork Avian comes into play, put four +1/+0 counters on it. At the end of any combat in which Clockwork Avian is assigned to attack or block, remove a counter.
o
X, oc
T: Put X +1/+0 counters on Clockwork Avian. You may have no more than four of these counters on Clockwork Avian. Use this ability only during your upkeep.

Can attack or block even if it has no counters.

If the ability to add counters resolves when there are already the maximum number of counters on it, any counters over the maximum are simply not added.

Loses a counter even if it is affected by a Fog-like effect which prevents it from dealing damage.