What are some alternatives?
When comparing FontStruct and BirdFont, you can also consider the following products
FontForge - Free (libre) font editor for Windows, Mac OS X and GNU+Linux
FontLab VI - font creation and editing, including variable fonts and color fonts
FontCreator - This professional font editor allows you to create and edit TrueType and OpenType fonts.
TruFont - TruFont is a font-editing application written with Python3, ufoLib, defcon and PyQt5.
Type light - With Type light you can design, edit and convert OpenType, TrueType and PostScript fonts.
Glyphs - Make everything from dingbat webfonts to full-fledged text typefaces in the most Mac-like font...