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