Sudden Aggressiveness
Flyers don't go out of the way to bite someone just to be "mean." There could be several reasons for this happening. The most common cause is an odor. Something on the hand that smells like food. Or it could be an odor the flyer doesn't like. A lot of flyers do not like certain soaps, perfumes, lotions, colognes, hair spray, etc. It can cause them to attack or bite owners who have had their flyer for years and had never been bitten. Flyers have a very keen sense of smell.
Females in heat and those pregnant often develop "an attitude" which includes biting and generally "obnoxious behavior." It will pass.

Biting
An aftershave or other smells can turn a flyer "off". The "way" you take the flyer from his nest might contribute to the problem. Take a bit of bedding fluff with the flyer, especially an adult, and drop the whole bundle down your shirt. "Grabbing" at a flyer can make they very nervous ... remember, they are prey animals -- and see humans as a predator. Yell "NO!" if bitten and hold his face to your chest so he cannot bite or else time him out in a carrier. A loud hiss when your pet looks ready to bite can also work.

Ear Biting
If things get very very bad, you may have to paint your ears with Bitter Apple. Just a side note...never, ever stop giving the fliers hard shell nuts, UN-cracked. Trimming or filing their teeth is also an ugly situation.

Water Discipline
An excellent discipline aid for many undesirable animal behaviors is a small spray bottle of water. Set the tip to fine mist and have it handy so you can squirt them IMMEDIATELY after the undesired behavior. It's very effective and relatively harmless, but avoid spraying their ears or face. A small squirt combined along with a firm "no" and they understand that you are displeased.

Another technique that might work (if the squirrel is tame enough). While you hold him, firmly but GENTLY wedge the TIP of your finger into his mouth, and hold his mouth open wide with your finger for a second or two. Be gentle but firm. The idea is to teach him to associate a slightly uncomfortable feeling with your finger being in his mouth. Try this a couple of times a day for a few days.

A combination of techniques can best of all. Behavior problems are kept to a minimum when your let your pet know you are not only in LOVE with the little sweetie, you are in CHARGE.

Won't eat veggies
Buy some baby food spinach, mix it with berry yogurt and a tiny tad of peanut butter. Add a bit of granola, roll into a little ball. You can also crack open a nut and mash a bit of cooked kale into the nut's crack. Be creative. Try plant matter, such as Spanish moss, likens, mosses, dandelion flowers and tender new leaves, rose blossoms, honeysuckle flowers and fern fiddle heads.

Stereotypic Behavior
The words "ritualized" and "clockwork-like" have been used to describe these behaviors. The salient points are that the behaviors:
a) do not occur in the wild
b) are repetitive
c) are apparently functionless.

Back flipping is a stereotyped behavior akin to feather plucking in pet birds and other problems due to excessive boredom. Some flyers can become so accustomed to doing back flips that they cannot run in a straight line. Some have become so habituated that even running across the floor, they will run a foot jump straight up like bouncing off something, run a foot, jump straight up, etc.
Also, if you watch a squirrel that really does a lot of back flipping, they will get dizzy. When they stop their head will nod for a bit and they sometimes start falling over like they are off balance. Running a circular track pattern around a cage isn't much different from back flipping. Running in a wheel isn't much different then running in a straight line. They won't get dizzy or lose the ability to run in a straight line.
Encourage them to use a running wheel. Place them in it occasionally and they will get the idea. They have lots of excess energy that needs to be burned off. If they don't use a wheel, they may develop habits like nonstop back flips, running in circles, etc.

Trembling/Vibrating
New owners are often frightened when they discover their flyer trembling when being held. This is really NOT a problem, but often ultrasonic communication that is out of human hearing range. Another reason is "bruxing" in which the flyer is grinding it's teeth. This is a natural phenomena in flyers who need to keep their teeth ground down.

Nails Too Long

1. Put a flat rough stone under the food dishes. They have to walk across the stone to get to the dish.
2. There are cement perches for birds that help keep the nails short
3. You can also get a nail-o-matic that fits in a Wodent wheel http://http://www.sugar-gliders.com/glider-nails.htm Less expensive way is to cut a strip of fine sandpaper and line the inside of the wheel.
4. Put hardwood branches in the cage and there is usually no problem with long claws.
5. As a last resort ... the nails can be trimmed. If you look at them the tip is a lighter color and that can be trimmed off.
   

Overgrown Teeth
Flying Squirrels don't make very loud noises unless they are cracking nuts in the middle of the night. Make sure flyers have wood and nuts in their cage to gnaw on. This helps decrease the size of the incisors.
One reason for overgrown teeth is a jaw problem which causes him to not want to gnaw. You will have to consult a vet for this. The second reason is just being lazy because he always had something else to eat and didn't HAVE to gnaw open nuts. Try withholding all softer foods and only give him dry corn, rodent blocks, Zupreme monkey chow and nuts. If he only eats the monkey chow, use only Rodent blocks that contain most necessary vitamins and nutrients. They are about 1/2" square or rounded blocks sold at most pet stores. Zupreme may be better nutritionally but not as hard.
A chinchilla lava block and/or a peanut/lava block (found in bird section of pet stores) may invite him to chew. Some flyers will chew those cement bird perch's.

Chewing Things They Shouldn't
The only downside to a Flying Squirrel is that they will chew things. If it gets away from you and gets into the attic (for example), it can chew through wires. They are also able to chew on wood, plastic, and fabrics. If your Flying Squirrel isn't bonded, be sure it is restrained in some way to prevent escape.  Gnawing is usually caused by needing the teeth worn down but can be from boredom or hunger. Make sure your flyer has a good supply of hard shelled nuts in the cage. Hazel nuts are good because they are small but have a hard thick shell to get through.  Some have found it helps to keep my flyers from chewing off limit things by placing live tree branches located throughout their out of cage area. Some even provide a branch that has been dead long enough to grow rather soft and gather bugs. Some flyers will forget about everything else. Of course, most people might not want 'buggy' logs in their home, but the bugs do not live long enough to cause a problem. Flyers think this is about the best banquet going.

Bitter Apple Vs Fooey!
If you are going to try and prevent chewing or sucking by using a product like Bitter Apple, know that it is like 26% alcohol (beware of using it on any tender skin) while Fooey! has no alcohol, no odor, and is safe to apply directly onto animals including birds and all rodents. Leah sprayed a bit on her finger and says it is NASTY, HORRIBLE stuff! She has high hopes for this stuff! Report right from the Fooey's mouth.