Here is the full list of all 13 Flock achievements.
KNOCK OVER 50 FENCES WITH A COW (AGGREGATE TOTAL)
USE 3 LOVE PATCHES IN 3 MINUTES
COMPLETE A LEVEL FEATURING THE NIGHTTIME PREDATOR WITHOUT LETTING IT EAT ANY ANIMALS
FLY AND LAND 75 CHICKENS (AGGREGATE TOTAL)
FLOCK 25 ANIMALS AT ONCE
COMPLETE FIRST LEVEL
ABDUCT 20 ANIMALS (AGGREGATE TOTAL)
RIP UP 50 TREES (AGGREGATE TOTAL)
PLAY THROUGH ONE USER-CREATED SINGLEPLAYER LEVEL AND ONE USER-CREATED MULTIPLAYER LEVEL
GET A PERFECT ABDUCTION ON ALL LEVELS, SINGLEPLAYER AND MULTIPLAYER
COMPLETE 10 CROP CIRCLES (AGGREGATE TOTAL)
KILL 10 CHICKENS WITH ONE THROW OF A BOULDER - STRIKE!
USE COWS TO MAKE 50 PILES OF POO (AGGREGATE TOTAL)