Walking Stick -- tree or shrub?

Above, from left: catkins on display, leaf catching the sunlight, trademark curly corkscrew of branches

I never noticed the 'walking stick' tree/shrub until a friend drew my attention to it yesterday. Obviously 'walking stick' is a misnomer as something so twisted wouldn't make good raw material for a walking stick. But the corkscrewing branches do add great interest to a winter landscape when the tree has lost its leaves. 

The 'walking stick' has various names: contorted filbert, corkscrew hazel and Harry Lauder's walking stick. There is also lack of consensus as to whether it is a tree, a 'tree-like shrub' or a deciduous shrub. One point of agreement is that they all bear yellow catkins that start in late autumn and get bigger in the spring. This shrub, which can grow to 10 feet tall, is a relative of the birch and will thrive in sun or partial shade.