(２１)
Make valley-folds at the dotted lines.
(２２)
Make valley-folds like this picture.
(２３)
Then make a mountain-fold at the yellow line.
If you pinch it from top while putting your finger under the bird-base, it will get easier to make a mountain-fold.
(２４)
Make a mountain-fold like this picture.
On the opposite side too, make a mountain-fold in the same way.
(２５)
After making a mountain-fold on the opposite side, it looks like this picture.
(２６)
Then move both sides in the direction of the arrows while pushing the circled parts a little inwards.
(２７)
This picture is halfway through.
(２８)
After making shape like this picture, move the top and bottom in the direction of the arrows.
(２９)
Move the top and bottom like this picture.
(３０)
The bird-base is finished.
Then check whether the other square is caught on the bird-base or not.
(３１)
Take two head/tail and move each of them in the different direction.
(take two head/tail while referencing the next picture.)
(３２)
Pinch the circled parts and move each of them back and forth.
(３３)
If the other square move when you move two head/tail, the other square is caught on the bird-base.
If the other square is caught on the bird-base, you will not be able to make this model well.
Arrange or adjust the paper so that the other square is not caught on the bird-base.
If the other square is not caught on the bird-base, proceed to the 40th step.
(３４)
Arrange or adjust the paper so that the other square is not caught on the bird-base.
Move the other square in the direction of the arrow like this picture.
(３５)
Then move the other square in the direction of the arrow as well.
(３６)
After moving the other square, take two head/tail and move each of them in the different direction again in order to check whether the other square move or not.
(３７)
If the other square move again when you move two head/tail, there is a high possibility that the other square is caught on the bird-base because the corners of the other square are turned up like this picture.
If the corners of the other square are turned up, make creases so that the corners are turned up a little.
(３８)
Make creases so that the corners are turned up a little like this picture.
(３９)
Then take two head/tail and move each of them in the different direction again in order to check whether the other square move or not.
If the other square move again when you move two head/tail, make this model once as it is and check the cause.
(４０)
After checking that the other square is not caught on the bird-base, put the other square back like this picture.
