and what types
cockatiels are great companions as they are flock birds so it really depends what you want of them! if you would like to train and have relationships with them, i personally suggest getting two as they will still be somewhat interested in you. if you get four, you may find that its more of an aviary and the birds will be a lot less inclined to come out, play with you, and a bit harder to train. With two, you will still be considered a part of their "flock" with four, they already have their flock!
It depends whether you want colours on the bird or not. Normal Greys come with yellow heads and red cheeks whereas whiteface are usually just black and white. I prefer whitefaces (what i have) as they all look different! They usually have different markings in different places and they are a bit easier to tell apart from one another, they also have pink feet and beaks rather than the grey. Pearls and pied are beautifully marked birds but have a look at this link of the different types and choose one you really like:) http://members.optusnet.com.au/~ geoffwatts1/Cockatiel_Mutations.html
I think you should get two that way you could breed them if you want and they'll keep each other company:) and you should get the Normal Grey
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INFORMATION bout normal grays:
Cockatiel Normals have the same coloration as cockatiels commonly found in the wild in their native Australia. They have dark grey feathers with some patches of white on their wings. Their faces may have some yellow, and they have the orange cheek patches that most cockatiels have . Their cere, bill, and feet are also grey.
Birds are very hard work-I have one. So if you've never had a bird and don't know how to handle them very well but you are still going to get some then get to. If you know how to handle them then you should get 3 or 4. If you get 4 then get a large cage and some toys and some ear plugs. and get an African gray parrot they are very loving.
If they are just to be cage pets just get two ... four may experience more dominance issues and fighting and will certainly be harder to tame and a hell of a lot noisier. I have two cockatiels and I love them dearly but I certainly would NOT want more of them!
I think two would be great. They can be very loud if you have more than that. To male sure you have a male or female the male has an bright orange dot on his cheek .
it depends on how large your cage is. I would get two and let them have more space then to try and cram 4 birds into the same space. Go for quality of life not quantity of birds.
two. and for what types .... do some research.