This feature requires the plus version of Birdfont.
You can export many weights of a font by first creating a variable font and
then specify the parameters for the fonts you want to generate from the variable master font.
1. Create a variable font.
2. Open the glyph editing tab or the kerning tab and adjust the variable slides you have until it looks good.
3. Go to export settings. Menu → Import and Export → Export settings.
4. Click on "Add static export from variable font."
5. Give this style a name.
6. Repeat for all the styles you want to export.
7. Click on "Export All" at the bottom of the page.