The most important part of Flock! is understanding your animals and the things you can do to get them into your UFO. Read these tips to see the best ways to handle your herd.
Sheep are your bread and butter. They are easy to move. They form flocks well. They try to naturally avoid ledges. Sheep in general are fairly easy to control. The only trick is shrinking them. Getting them wet will cause them to shrink. These little sheep are more erratic in movement, but they can squeeze under fences. Nothing too special about sheep though.

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Cows are an interesting bunch. Their main ability is to charge when pushed. Keeping them pressured will cause them to rush forward and knock over all the fences in their paths. They will also naturally knock down any fence that gets in their way. This is useful, and in fact required, for several puzzles. The only annoyance is that they are very sluggish. The cows in Flock seem to move and turn fairly slowly, making deliberate control very trick. If you ever accidentally pass them up, you will have a heck of a time repositioning yourself.
Pigs are somewhat annoying. They are the pinballs of the game. The biggest problem is that they start to roll really quickly when pushed for any amount of time. If they hit a level bumper, then they will ricochet off in another direction. It takes very careful control to get them where you want. They have a natural tendency to roll toward the piles of manure in the game field. This is useful in a few puzzles. They will stop to roll around in it though, which can hurt your time.
Chickens are another interesting group. They are very quick and form really tight flocks usually. Most of the puzzles with chickens are very simple. You usually just need to take advantage of their ability to glide. If you push a chicken off of a cliff, they will fly forward for awhile. This is quite useful in a number of puzzles, as it allows you to take shortcuts and get a little creative with pathfinding. It also lets them hop over pits (usually).
All of the species have a baby that sometimes comes into play. The baby is usually just a small and quick version of the larger one. They scare a bit easier, but tend to flock very well around an adult figure. The general danger is that it is quite easier to accidentally scramble the flock.
All of the species also have a female. This figure is usually pink with bows. Males will form a line behind the female and follow her right up to the abduction beam. The only problem is that any break in the line will cause all of the others to stop following. This means that you'll need to limit your line to roughly 5 animals. The female can also breed with a male on certain love squares in the game. This produces a flock of babies and is often a key part of the puzzle.