Would love to attempt a Big Bird dressed up as Robin for a comic convention next year.  Any thoughts on how the neck and head are done regarding support?  Guessing the body would be something like a wire support surrounded in foam.  hmmm...might have just answered my own question :D

The head is held up and manipulated by one of the puppeteers arms.    Body could be a layer of fabric, foam and then outer covering that was then held in shape by plastic tube hoops. You can find plastic tubing at the hardware store in different gauges.

This should give you an idea of what I am talking about. http://entertainment.howstuffworks.com/muppet4.htm

Thanks for the link So, using plastic tubing, is the body collapsible?

So, after some research I can see that I have chosen an interestingly difficult project.  I also can see that I will have to do some modifications to make it more...wearable friendly for longer duration.  For example, the 4 1/2 pound head.  Going to attempt to have it supported so that I can bring my arm down for resting while still being fully movable.  Meaning, this Big Bird may have gained a bit of weight to give me a little more room inside.  Since the body is feathered, probably going to play around with multiple sections over the longer piece I saw in a behind the scenes video.  But first things first, going to see if I can get the beak carved out recognizable. 

It may be worth finding fursuit or mascot-building communities - they would have more ideas of how to build something that you can wear but is also light and breathable.

The bulk of the weight in the head is pretty much the beak, the head itself is fairly small, but the feathers make it look much rounder and larger. I believe the body itself also is a screen material designed to be light-weight covered in feathers.

The one they use for the live shows at Sesame Place is built much different, the head is not the puppeteers arm, but instead rests on his (her) shoulders. For extrended usage, you may want to look into that, perhaps with cables for the mouth and head turns?

It may be worth finding fursuit or mascot-building communities - they would have more ideas of how to build something that you can wear but is also light and breathable.

That isn't a bad idea for the body, I will have to look. 

The bulk of the weight in the head is pretty much the beak, the head itself is fairly small, but the feathers make it look much rounder and larger. I believe the body itself also is a screen material designed to be light-weight covered in feathers.

The one they use for the live shows at Sesame Place is built much different, the head is not the puppeteers arm, but instead rests on his (her) shoulders. For extrended usage, you may want to look into that, perhaps with cables for the mouth and head turns?

I will have to keep that in mind, was wondering if I should try to rest it on the shoulders for support.  Going to attempt to use elastic to control how open the mouth is in a resting state, though I will keep cables in mind if I run into problems.  Are those fairly easy to setup?

Yes if you use plastic tubing the body can be collapsible. You build the body with the combination of lining, foam and then outer covering that I described. Make your lining of mesh like you see in football jersy's. http://www.fabricandsewing.com/Product.aspx?ProductID=133&&AspxAutoDetectCookieSupport=1  If you google "football jersey fabric" you should come up with more examples and places you can buy it.  You use this so the costume is breathable and washable. In fact I've actually seen costumes that use this fabric on what we call a "pod" with it being the inner and outer layers with the foam between. They then make a outer fur or other fabric costume that can be pulled over this. Once you get this pod built you use the tubing to make "hoops" that can go inside it to hold out the form.  These can be held in place with either fabric channels sewn on the inside or Velcro loops. Same idea as building a hoop skirt except that you make it so you can take the hoops out an the tubing holds it form better then regular boning. Two to three hoops should be enough to hold the form of your body.

Thanks Shawn, that makes sense.  In regards to the head, I am thinking of enlarging the glorified sock pattern and making it out of 1/2 inch or 1 inch foam.  With some extension, this would be the beak.  Then, as Animal31 mentioned, round it out with the feathers.