It is not normal for birds to fall off their roost unless they are sick.
The things that come to mind that could be causing this are....
Marek's disease..... they are the right age to get it and they have perhaps been stressed by the move outside and stress is a trigger for a Marek's outbreak. It can cause neurological symptoms like loss of balance.
There is no recognised treatment and it is often fatal but a good poultry supplement like Poultry Cell or Nutri Drench will help to support their immune systems and give them a chance to fight it. Falling off the roost or getting stuck in odd places is a common occurrence with Marek's, as it comes on quite suddenly. They can sometimes fully recover as quickly as they became debilitated, but they will be prone to future attacks at times of stress and will be carriers for life. It is caused by a virus that lies dormant for long periods and they shed the virus in dander dust which is inhaled by other birds to become infected. It is an extremely common and widespread disease and young birds are particularly vulnerable to it.
Crop impaction..... if they have recently had access to grass when they haven't previously, they can gorge on it and if the grass is longish, it can get tangled up in their crop and slow the rate at which food can pass into their digestive tract. Birds with an impacted crop will often snake their head and neck to try to adjust the crop contents.
To check for this, remove feed at night. Check their crops when they go to roost and check them again the next morning before they have access to food. Their crop should be full at night and empty in the morning. If it is not empty in the morning then this is most likely your problem. The birds will also start to lose weight and their breast bone will start to feel sharp under the skin. The lack of food going through their system will make them weak and the weight of their crop will unbalance them. The treatment usually starts with removing access to food and just giving them water with a poultry supplement as mentioned above and feeding them a couple of teaspoons of coconut oil each day and massaging their crop for 5-10 mins 3-4 times a day to try to break up the mass. It may take several days to break it up or they may need other treatments or even surgery if it still will not shift.
Coccidiosis.... having just been exposed to the outside, they may be suffering from an overgrowth of coccidia in their gut. These are little micro organisms that are everywhere in the soil but can flourish in the gut of certain animals (usually juveniles) for a variety of reasons. Coccidiosis is often associated with warm wet conditions.
Birds suffering from it will usually stand around hunched and sleepy and will have loose or bloody poop. They can become weak and wobbly as a result. Treatment is Corid which you will find in the cattle section of your feed store. Dosage is 1.5 teaspoons of the powder or 2 teaspoons of the liquid form in a gallon of water, made fresh each day for 7-10 days and that needs to be their sole source of water to ensure they get enough of the medicine...... so no access to muddy puddles or other standing water.
Chemical poisoning.... as
@Little Jerry Seinfeld suggests, they may have ingested grass that has been treated with chemicals that is affecting their neurological system. You would need to prevent further access to it and I believe activated charcoal can be beneficial in absorbing residual toxic chemicals from their digestive tract. Again a vitamin supplement, particularly vitamin B complex may be beneficial to support their nervous system but hopefully with time the effects will wear off.
There may be other causes that I am not aware of, but those are the commonest and most likely things that I can think of which relate to your circumstances, based on the information that you have provided and Marek's Disease is probably the most likely of those. Without being able to see your sick birds or your set up, it is very difficult to be sure of anything though, so veterinary advice might be more beneficial.
Good luck with them
So I got 2 chickens that use to act normal till I let them outside and then they acted a little funny. They shake their head , walk dizzy , and can't balance on a perch but they eat ,lay, drink, and act fine i am worried. What could it be?
