If Glint Hawk Idol and another artifact enter under your control at the same time, Glint Hawk Idol's first ability will trigger. You may have it become a creature, but since it will be affected by "summoning sickness," it won't be able to attack that turn.
The creature type "Bird" was inadvertently omitted from each of Glint Hawk Idol's abilities. It has received errata to correct this omission; when either of its abilities resolves, it will become a Bird artifact creature.
For example, say one of Glint Hawk Idol's abilities has resolved and it's a 2/2 creature. You cast Giant Growth targeting it. It's now 5/5. Then Diminish ("Target creature becomes 1/1 until end of turn") is cast targeting it. Once Diminish resolves, Glint Hawk Idol would be 4/4. Activating Glint Hawk Idol's last ability at this point would make it 5/5 again until end of turn.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Dave Allsop
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If Glint Hawk Idol becomes a creature before you begin a turn with it under your control, it will be affected by "summoning sickness."
If either of Glint Hawk Idol's two abilities resolve while it's a creature, that ability will override any effects that set its power and/or toughness to another number, but effects that modify power and/or toughness without directly setting them will still apply.
