Birds should be wormed regularly - How regularly depends on a number of factors. Some wormers are more of a preventative than a wormer so need to be used monthly. Whereas some will clear the bird of worms if they are carrying them. If your birds are confined to a small area they have more chance of picking up a worm burden we would therefore recommend worming more frequently (every 2-3 months.) If they have access to fresh pasture that is regularly mowed and clippings taken away then worming every 6 months may be sufficient.
We use Flubenvet/ Flubenvet is in powder form which needs mixing in the birds feed.
To prepare - First of all measure out how much feed a pen is going to eat in a week and then weigh it! Add a small amount of kitchen oil (vegetable / Olive oil etc) and mix until just very very lightly coated. You then measure the amount of Flubenvet you need for that quantity and mix. The oil allows the powder to stick to the pellets otherwise it all drops to the bottom of the feeder and not in the birds mouth! There is no egg withdrawl.
If you want a natural 'wormer' then Verm-x for poultry can be used. Vermex involves mixing a measured amount of pellets in with the poultry feed - the idea being that the birds that weigh more will eat more so they get the correct dosage. The product also comes in a liquid form which can also be added to the water, but you do need to ensure that they can only drink from the water containers and no other 'untreated water' otherwise they will not receive the correct dosage. Verm-x is used every month as a preventative - it is not guaranteed to be able to remove existing worms - just to prevent them. Again no egg withdrawl.