Why are hazelnuts called filberts?

The most commonly accepted explanation is because hazelnuts mature on or around St. Philibert’s Day, August 20. Other historians believe the term filbert derives from the German vollbart meaning full beard, a reference to the appearance of the husked shell. Although the current definition of filbert tends to refer to commercial cultivated crops of hazelnuts, the terms hazelnut and filbert are generally used interchangeably. Hazelnuts are also known as cobnuts in some areas. Other experts claim these are all different varieties of the nut, but once shelled, they are quite difficult to tell apart.

A Wikipedia entry says that the filbert is a species of hazelnut.