The Stirge does nothing other than ongoing damage to its target while it has its target grabbed.
The updated StirgeDDI from the Monster Vault clarifies this.
The MM Strige's Bite attack reads:
+6 vs. AC; 1d4 damage, and the target is grabbed (until escape) and takes ongoing 5 damage until it escapes. An attached stirge doesn’t make attack rolls while grabbing a target and gains a +5 bonus to its AC and Reflex defenses.
The Monster Vault's Bite attach reads:
Requirement: The stirge must not be grabbing a creature.
Attack: Melee 1 (one creature); +6 vs. AC
Hit: 1d4 + 5 damage, and the stirge grabs the target (escape DC 12). Until the grab ends, the target takes ongoing 5 damage.
And a trait is added indicating:
While the stirge has a creature grabbed, the stirge gains a +5 bonus to AC and Reflex