Zombie Raven Swarm CR 3
Neutral Evil Tiny Undead (Swarm)
, Senses ; ; Darkvision 60'
AC 11, touch 11, flat-footed 10 (+1 Dex)
hp 47 (HD); DR 5/slashing; 1/2 damage from slashing or piercing weapons
Immune swarm immunities, undead immunities
Weakness +50% damage from area-affect spells or effects
Spd 10' (2 squares), fly 40' (clumsy)
Melee Swarm (2d6)
Space 10 ft.; Reach 0 ft.
SQ Single Actions Only (Ex)
Environment Temperate forests
Organization Solitary, pack (2-4 swarms), or infestation (7-12 swarms)
A maddening, flapping wall of beaks, feathers and dead eyes squirms through the trees with a cacophany of guttural cries. It leaves a trail of self-torn clumps of cold flesh and feathers.
Single Actions Only (Ex) – Zombies have poor reflexes and can perform only a single move action or attack action each round. A zombie can move up to its speed and attack in the same round, but only if it attempts a charge.
Distraction (Ex) – Any living creature vulnerable to a swarm’s damage that begins its turn with a swarm in its square is nauseated for 1 round; a Fortitude save (DC 14) negates the effect. Spellcasting or concentrating on spells within the area of a swarm requires a Concentration check (DC 20 + spell level). Using skills that involve patience and concentration requires a DC 20 Concentration check.
As far as I know, there's no Swarm template, so I picked a swarm HD that seemed to be appropriate for a 1/2 HD creature. The rest is applied easily enough from the swarm subtype information. CR for swarms is approximately half HD. The base creature is Zombie Raven CR 1/8.