Here is the full list of all 13 Flock achievements.
FLY AND LAND 75 CHICKENS (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
RIP UP 50 TREES (AGGREGATE TOTAL)
PLAY THROUGH ONE USER-CREATED SINGLEPLAYER LEVEL AND ONE USER-CREATED MULTIPLAYER LEVEL
USE 3 LOVE PATCHES IN 3 MINUTES
GET A PERFECT ABDUCTION ON ALL LEVELS, SINGLEPLAYER AND MULTIPLAYER
KNOCK OVER 50 FENCES WITH A COW (AGGREGATE TOTAL)
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)