Bird base is an extension of Square base. Since I do not want to repeat the same thing, we will start with a Square base here initially.
Start with a square base.
Here the open side is at the
Prepare for two valley-folds.
Apply two valley-folds according to the step #2.
Prepare for another valley-fold.
Valley-fold according to the step #3.
Prepare to unfold three previous valley-folds.
Unfold all three valley-folds.
Let's begin so-called petal-fold.
Lift the bottom corner upwards.
As you push the corner up, bring the edges in towards the middle and flatten them.
Now you completed the petal fold.
Prepare to turn the model over.
Turn the model over.
Now, the situation is the same as the step #2.
Prepare to repeat the steps #2-9.
Repeat the steps #2-9.
The bird base is now completed.