At first we weren't quite sure what to do with our 8-year-old's request to be a bird in a cage but we've got a policy in our family that if you really want to be something for Halloween we'll do our very best to come up with a costume.
The bird part we were okay with. In some ways, because we would only be dealing with the top half of her body (the rest being hidden under the cage), our job was a little simpler. We built a padded "shirt" which fastened up the back and then spent the next few days glueing hundreds of individual feathers to it. The head piece, which would tuck into the shirt, was built in a similar fashion.
We knew the cage had to be kept as light as possible but also rigid, stable, and able to stand up to a fair amount of abuse. We began with a circular base made of two layers of 1/4" foam core glued together. To that we attached a ring of bars made of PVC plumbing pipe. To get the shape we wanted these were partially stiffened with a wooden dowel inserted into the pipe. Each bar was inserted into a hole drilled into the foam core and held in place with hot glue. The upper ring was also foam core which was hot glued in place. Finally, all the bars were bent over and inserted into a styrofoam ball to cap it off. The whole assembly was then sprayed white.
To support the cage we made a harness from an old backpack which attached to the cage base with nylon straps. For additional stability we also added a piece of 4" upholstery foam under the cage which as hidden by the skirt.
We added the bird's feet and tail directly to the bottom of the cage and covered the rest of the base in newspaper. One of those clear, plastic pegboard bins served as a food dish for the bird and also a handy place for our daughter to put her candy. To complete the look, we added a bird toy from the pet store dandling above her head.