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Brooklyn Origami Circular Bird 3
[3] Starting at point X, fold the lower two thirds of the right half circle to the center crease until the curved edge of the paper touches the bottom 1/4 mark. See Figure 4 below to see how the paper should look once this step has been completed.

Brooklyn Origami Circular Bird 4
[4] The result of step 3. Note that a straight edge -- edge XZ -- has been created.