Here is the full list of all 13 Flock achievements.
RIP UP 50 TREES (AGGREGATE TOTAL)
USE 3 LOVE PATCHES IN 3 MINUTES
KNOCK OVER 50 FENCES WITH A COW (AGGREGATE TOTAL)
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
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)